## Filling Gasoline Tank To The Brim To Compute MPG

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Topic Author
Leesbro63
Posts: 7429
Joined: Mon Nov 08, 2010 4:36 pm

### Filling Gasoline Tank To The Brim To Compute MPG

I'm wondering how to get an exact MPG reading. It seems that the only way to be sure that the gas tank is filled to the exact same point is to fill it to the brim where you can actually see the gasoline in the tube, just to the point before it dribbles out onto the paint. But I read this hurts the carbon cannister...yet I've never actually known anyone or even known anyone who has known anyone with this problem. How do you get accurate MPG readings?
dukeblue219
Posts: 2184
Joined: Fri Jan 29, 2016 12:40 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

If you use the same type of gas pump and stop immediately at when the pump clicks off, shouldn't that be close enough? It's not full but it'll be around the same level at each fill-up for comparison.

I just use the car's readout, though.
sureshoe
Posts: 657
Joined: Tue Jan 15, 2019 4:26 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

There really is no reason to top off gas like this.

First off, I am assuming most gas dispensers have a vapor recovery system, and you're actually choking back some of the fuel, so your "to the brim" calculation is wrong regardless.

Just fill your tank the first time.
Drive until low, refill. Say you went 400 miles and refilling to the first "click" takes 20 gallons.
Do the same, this time 408 miles on 19.5 gallons.
Rinse repeat.

You're going to be in a range, you're never going to get an "exact" measure.
Point
Posts: 433
Joined: Mon Jul 10, 2017 9:33 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Reset the trip odometer on each fill up and you’ll
Be close enough.
mrc
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Joined: Sun Jan 10, 2016 6:39 am

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Use the same pump. Fill to the first click, then more slowly until the second click. Stop. Repeat over several tank fulls. Close enough. If you keep a long and to this for a season you'll be as close as practical.

If you fill to the brim, besides mucking up the pollution control, you could lose fuel to expansion overflow with temperature increase.
By the time you know enough to choose a good financial adviser, you don't need one. | bogleheads.org is my advisor: The ER is 0.0% and the advice always solid.
Jags4186
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Joined: Wed Jun 18, 2014 7:12 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

There’s no real practical way of doing this.

1) drain the tank, fill it up with a precise amount of fuel, run till empty, calculate MPG
2) weigh a gallon of gas, fill up the car, weigh the car, run the car until the tank is say 1/4 filled, weigh the car again, difference in weight is the usage of fuel, and calculate your MPG that way.

I just go by the onboard computer.
tibbitts
Posts: 15736
Joined: Tue Feb 27, 2007 6:50 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Jags4186 wrote: Thu Nov 18, 2021 3:42 pm There’s no real practical way of doing this.

1) drain the tank, fill it up, run till empty, calculate MPG
2) weigh a gallon of gas, fill up the car, weigh the car, run the car until the tank is say 1/4 filled, weigh the car again, difference in weight is the usage of fuel, and calculate your MPG that way.

I just go by the onboard computer.
I think the OP is way too concerned about precision beyond what has any practical value, but I will say that my onboard computer has always been about 10% over what I'm actually getting. So, not even close.
iamlucky13
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Location: Western Washington

### Re: Filling Gasoline Tank To The Brim To Compute MPG

It depends how accurate you want to be.

When I was working with a vehicle manufacturer, for official tests, we actually had a test tank we would plumb in, so that we could remove it to be weight before testing and again after testing. I wasn't in charge of those tests, so I don't know the actual calculated accuracy, but I'm sure it was better than 1%.

If you go by volume, the result will be affected by temperature. There is a 3% difference at 30 deg C compared to at freezing. The pump itself should meter in the ballpark of 0.5% accuracy by volume, but that doesn't answer the question of whether you put in exactly as much as you burned since the last fill-up.

If you use the same pump and stop filling when it clicks off, you will probably be within 5%, although I have not validated that. Do you really need to be more accurate than that? How much difference does it make to you if you calculated 27.5 mpg for one tank, but were actually getting 26.1 mpg?

If you monitor over multiple fill-ups, the inconsistency in how much you fill the tank each time averages out, and the calculated value starts to approach the pump accuracy. I'm seeing very consistent 39 to 41 mpg per tank on my car in my regular use (exceptions if I take a road trip or get stuck in really bad traffic). That 2 mpg variation doesn't bother me, because I'm highly confident if I total up my miles over multiple fill-ups, and total up my gasoline purchased, I will get a more accurate answer.

General engineering advice: do not get carried away chasing significant figures unless it gives a benefit proportionate to the labor and cost involved.
Jags4186 wrote: Thu Nov 18, 2021 3:42 pm I just go by the onboard computer.
If I remember right, my onboard computer reported 2-3% high over the average of 3 tanks last time I checked. I consider that pretty good, but it can be off by more than that.
Topic Author
Leesbro63
Posts: 7429
Joined: Mon Nov 08, 2010 4:36 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

mrc wrote: Thu Nov 18, 2021 3:36 pm Use the same pump. Fill to the first click, then more slowly until the second click. Stop. Repeat over several tank fulls. Close enough. If you keep a long and to this for a season you'll be as close as practical.

If you fill to the brim, besides mucking up the pollution control, you could lose fuel to expansion overflow with temperature increase.
This is good. I like this. Thank you. I'll do this. It reduces the risk of significant underfilling by a click that is premature, but doesn't go to the extreme of topping off. I might even go for the 3rd click, which will be soon after the 2nd click unless the 2nd click is also premature. This makes sense.
David Jay
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Location: Michigan

### Re: Filling Gasoline Tank To The Brim To Compute MPG

mrc wrote: Thu Nov 18, 2021 3:36 pmUse the same pump. Fill to the first click, then more slowly until the second click.
Same idea for me. I go 3 clicks.
Prediction is very difficult, especially about the future - Niels Bohr | To get the "risk premium", you really do have to take the risk - nisiprius
LittleMaggieMae
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Joined: Mon Aug 12, 2019 9:06 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Why would you need an accurate to the ounce of gas MPG number? Are you one of those people who doesn't put gas in the car until it's "tongue is hanging out and it's coughing for water" like a cartoon character crossing a desert with an empty canteen???

Is your gas gauge broken and do live somewhere where gas stations are few and far between?

I drove a car with a broken gas gauge for 8 years and never accidentally ran out of gas.

The owners manual told me how many gallons the tank held. I assumed this number was accurate. The owner's manual also told me the expected MPG based on city or expressway driving. I suspected this number of being a little, um, fuzzy.

I did a little math - 12 gallons * 18mpg (from the owners manual) = 216 miles on a full tank of gas. My typical week driving was 28 miles * 5 days = 140 plus another 30 miles on the weekend - 170 miles more or less.

I topped off the tank on Sunday night. Made note of the odometer reading before I left the gas station and went about my week. The next Saturday I again topped off the tank. I made note of the odometer reading AND the amount of gas I put in the car. I was getting 20 miles per gallon.

I did that for a couple of weeks - and I was getting about 20 to 23 miles per gallon (depending on how much expressway driving I did on the weekend - and how many people I was carting around). I made an effort to refuel the car soon after driving 150 miles. I used the "trip odometer" which I could reset after every visit to the gas station to determine if and when I needed to put gas in the car. I verified the "accuracy" or "consistency" of the trip odometer during the handful of weeks after the gauge broke when I was tracking amount of gas and number of miles driven.

If my current vehicle's gas gauge broke - I'd do the same thing.
Last edited by LittleMaggieMae on Thu Nov 18, 2021 4:22 pm, edited 3 times in total.
TheGiantess
Posts: 21
Joined: Sun May 07, 2017 5:27 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Couldn't you drive until the gas tank warning light comes on, fill it up and then check your receipt to see how many gallons went in and then set your trip check or write down your mileage drive until the light comes on again? You will know your gallons and miles.
TG
Posts: 888
Joined: Thu Mar 01, 2007 7:46 pm
Location: MA

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Just be careful to not overfill on one of those extra clicks.

From Gasbuddy.com among many others.
"Your engine is made up of a complex, closed-circuit system that traps and burns the fumes from your gas tank. When you overfill your regular or diesel fuel tank too frequently, the EVAP system that is designed to process excess gasoline vapor (not the overflowing liquid) can be compromised, affecting your car’s performance and resulting in a check engine light, as well as a costly repair bill."
Regards | Bob
terran
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Joined: Sat Jan 10, 2015 10:50 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

I think filling up at the same pump at the same station until the automatic shutoff would be more than close enough. I wouldn't even be that concerned about filling up at different pumps and different stations and figuring they'll all be close enough to the same level. How much extra gas could possibly fit the the little filler tube with even an inch or two difference? Maybe 1/4-1/2 cup (1/64 - 1/32 of a gallon)? Say you had a small 10 gallon tank and went 300 miles for 30mpg, a 1/32 gallon difference would change that to 29.91mpg. A bigger tank would mean slight variations would make even less difference.
surfstar
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Location: Santa Barbara, CA

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Many readings over many tanks = accurate.

Fully thrown off with my plug-in hybrid now, except on long road trips. I'm data driven, yet have learned to move on
barnaclebob
Posts: 4623
Joined: Thu Aug 09, 2012 10:54 am

### Re: Filling Gasoline Tank To The Brim To Compute MPG

If you want any more accurate than the cars computer or filling until it clicks and dividing miles driven per gallon filled then youll need your own calibrated fuel flow meter and a good gps based mileage device.
TexasPE
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Location: Southeast Texas

### Re: Filling Gasoline Tank To The Brim To Compute MPG

barnaclebob wrote: Thu Nov 18, 2021 4:29 pm If you want any more accurate than the cars computer or filling until it clicks and dividing miles driven per gallon filled then youll need your own calibrated fuel flow meter and a good gps based mileage device.
Don't forget that % city vs highway mpg variability will overshadow the few ounces of gasoline between the 'first' and 'third' clicks.
At 20: I cared what everyone thought about me | At 40: I didn't give a damn what anyone thought of me | Now that I'm 60: I realize that no one was really thinking about me at all | Winston Churchill (?)
Bogle7
Posts: 1342
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Location: Mountain state

### Computing MPG

Keep a log book for your car.
I have. Since 1967.
Then, you can measure gallons consumed over thousands (tens of thousands) of miles.
I grew up with this as my father started in 1948.
Last edited by Bogle7 on Thu Nov 18, 2021 10:52 pm, edited 1 time in total.
Old fart who does three index funds, baby.
hunoraut
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

TheGiantess wrote: Thu Nov 18, 2021 4:19 pm Couldn't you drive until the gas tank warning light comes on, fill it up and then check your receipt to see how many gallons went in and then set your trip check or write down your mileage drive until the light comes on again? You will know your gallons and miles.
TG
1st the warning light is not precise.
2nd this doesnt address the unsolved piece of the puzzle: what is the reference mark to "fill it up"?

triggering the first nozzle click doesnt guarantee precision. (we can infer because the duration to the 2nd click varies). but over long enough timeframe, barring any other scientific method, it should be assumed to be good enough.

if you consistently use this same first-click method and compare it against the computer's readings, you can establish a pattern. the 2 sorta baseline eachother. most of my cars were consistently off by something like 2 US-MPG. some over, some under. last car was a BMW and its computer was on-the-nose.
cheese_breath
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

Your MPG will vary depending on the terrane you're covering and your driving habits.
When I used to travel a lot I'd just add up the gas for all my fill-ups and divide it by miles travelled. That's close enough for me.
The surest way to know the future is when it becomes the past.
Running Bum
Posts: 40
Joined: Sun Feb 10, 2013 2:00 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Over multiple tanks of gas, for each tankful do you drive the same overall speed, same elevation change, in the same traffic, same weather, etc? Even if I could figure out my precise MPG for a tankful, I might never get that same MPG for another tank in the life of that car. So I don't see the point. I just got a new car so I'll probably compare my calculated MPG vs the onboard computer to see how accurate it is, and I'll use the one click method but I won't worry about going to the same gas station, much less the same pump. Mainly what I want to see is how many interstate miles can I drive before I have to refill, and is the computer's "miles remaining" until empty reasonably accurate. The latter I guess I'll figure out by subtracting how much gas I actually put in from the 18.5 gallon tank size, and multiplying be a safe MPG estimate. My warning light is supposed to go on with 2.5 gallons left so that's another option.
Nate79
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

I would not go past 1 click. Maybe slow it down before getting to the 1 click. Some cars are very sensitive to overfilling and even going over 1 click is detrimental.
Mike Scott
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Joined: Fri Jul 19, 2013 2:45 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

My wife tracks every mile and drop of gas in her car. She just shrugs when I ask what she is going to do with all that information. I think (know) she just likes writing down numbers.
dukeblue219
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Joined: Fri Jan 29, 2016 12:40 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

I was wondering when the first BHer would come along that writes this stuff down in a little notebook. I remember my parents doing that in the 90s but it made no sense (and still doesn't) to me.

Why I'd ever care what my gas mileage is to more than +/- 5 MPG I'll never understand...
Not Law
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

Indication that something is amiss with the car's operation when the MPG drops.
Yefuy.Goje
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Joined: Fri Oct 29, 2021 3:29 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

***********

use the same gas station (and station number) for more accuracy.

1) fill the gas tank up just like you always do and let it stop automatically

2) record your car's current # of miles driven before you drive off of the gas station

3) come back in a later date and fill up the gas again. Let it stop automatically. You don't need to have empty tank. Record how many gallons you added to fill it up.

4) compute your car's gas milage by dividing the number of miles you dove since your last fill by the gallons of gas you add today.

Edit:. Reminded me of Archimedes
Last edited by Yefuy.Goje on Thu Nov 18, 2021 7:27 pm, edited 1 time in total.
jrmillions
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Location: Columbus, OH

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Your avg mpg will vary overtime regardless of what you do. There are many factors in the equation. How you drive-fast starts, slow starts, braking etc. Then mechanical factors like tire pressure (changes with the temperature), what kind of tires, type of gas, air filter clean? Etc. Driving uphill, downhill, gravel, asphalt etc. What the manufactures state is in a perfect environment under perfect conditions.
whodidntante
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

The ECU can be wrong, but it's a really good estimate. It knows exactly how much fuel is commanded per control cycle. Integrate that, and you have the denominator. It also knows how much distance has been traveled. Sure, there is error. But it's really good if the vehicle is in a stock configuration.

If you've noticed a drop in MPG, it's completely normal this time of year. Cold weather means the engine takes longer to come up to operating temperature. My own commute doesn't even allow time for the engine oil to reach normal temperature (yes, I have a gauge for that). I use an engine block heater when the temperature is Neptunian.

Also, check your tire pressure. The cold also causes a drop in pressure, which hurts fuel economy.

A scan tool will show you if you have excessive fuel trim, which would be worth chasing down. If you have no idea what that means, search around on youtube. I'm sure someone has made a video for it.
D. C. Pline
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

surfstar wrote: Thu Nov 18, 2021 4:28 pm Many readings over many tanks = accurate.

Fully thrown off with my plug-in hybrid now, except on long road trips. I'm data driven, yet have learned to move on
I second every word of this post.

I used to keep a spreadsheet. I could tell you the MPG over any given range of tanks: year 1, year 2, Spring, Summer, lifetime, etc.

Now I just drive.
Cheez-It Guy
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

dukeblue219 wrote: Thu Nov 18, 2021 6:44 pm I was wondering when the first BHer would come along that writes this stuff down in a little notebook. I remember my parents doing that in the 90s but it made no sense (and still doesn't) to me.

Why I'd ever care what my gas mileage is to more than +/- 5 MPG I'll never understand...
Guilty. And perhaps worse, I then transfer it to a spreadsheet for precision. I track my all-time bests and worsts as well as the vehicle's lifetime average. It allows me to brag about my fuel economy to anonymous nerds on the internet. I'm at over 44 MPG spreadsheet average for a 2-liter all-gasoline engine. My best full tank is just over 50 MPG, but that's exceedingly rare. It creates a game for me to see how much more economically I can figure out how to drive it. I have gotten an ECU calculated trip average of just over 60 MPG for my 20+ mile commute home on rolling terrain. It's fun. This is for a vehicle with EPA ratings of 26 City, 38 Highway, and 31 Combined. My conclusion is that most Americans drive very wastefully.
seawolf21
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

Yefuy.Goje wrote: Thu Nov 18, 2021 7:11 pm CORRECT ANSWER
***********

use the same gas station (and station number) for more accuracy.

1) fill the gas tank up just like you always do and let it stop automatically

2) record your car's current # of miles driven before you drive off of the gas station

3) come back in a later date and fill up the gas again. Let it stop automatically. You don't need to have empty tank. Record how many gallons you added to fill it up.

4) compute your car's gas milage by dividing the number of miles you dove since your last fill by the gallons of gas you add today.

Edit:. Reminded me of Archimedes

But the question is why the need to do this (determine MPG) to begin with?
BatBuckeye
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

The risk/reward of over filling your gas tank is not in your favor. It's okay to continue tracking your MPG, we all need something to occupy us.

It reminds me of a movie, the name escapes me, a neighbor of the main character obsessed over his MPG. The main character and his daughter, I believe, would at various and sundry times secretly add gas to the car. Maybe it was a VW bug. Where the neighbor just raved about his incredible mileage. Then there was one time when they siphoned gas out! His MPG seemed to plummet and the poor guy was on the verge of a breakdown. Makes me laugh.
You might be careful who you confess your MPG stories too. If you have that funny neighbor...
RedDog
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

tibbitts wrote: Thu Nov 18, 2021 3:45 pm
Jags4186 wrote: Thu Nov 18, 2021 3:42 pm There’s no real practical way of doing this.

1) drain the tank, fill it up, run till empty, calculate MPG
2) weigh a gallon of gas, fill up the car, weigh the car, run the car until the tank is say 1/4 filled, weigh the car again, difference in weight is the usage of fuel, and calculate your MPG that way.

I just go by the onboard computer.
I think the OP is way too concerned about precision beyond what has any practical value, but I will say that my onboard computer has always been about 10% over what I'm actually getting. So, not even close.
A frequent observation saying by aircrew members wrt to charted aircraft performance, “measure it with a micrometer, mark it with a grease pencil and cut it with a hatchet”.
andypanda
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

"It's okay to continue tracking your MPG, we all need something to occupy us."

I knew a guy a long, long time ago who used a stopwatch to calculate and record how much time he wasted every day sitting at stop signs and red lights.
tesuzuki2002
Posts: 1188
Joined: Fri Dec 11, 2015 12:40 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Leesbro63 wrote: Thu Nov 18, 2021 3:27 pm I'm wondering how to get an exact MPG reading. It seems that the only way to be sure that the gas tank is filled to the exact same point is to fill it to the brim where you can actually see the gasoline in the tube, just to the point before it dribbles out onto the paint. But I read this hurts the carbon cannister...yet I've never actually known anyone or even known anyone who has known anyone with this problem. How do you get accurate MPG readings?
What you are asking about is in the noise.... and won't matter. You do want to be consistent with your fill up.. So stop when it click off versus rounding up to the next dollar or gallon... this will throw calculations off.

Most vehicles have a big enough tank that it doesn't matter much... It's a bit more critical on motorcycle with a 4 gallon tank versus a normal 16 gallon tank. 1/2 Gallon on a 16 gallon tank yields a 3.1% calculation error. 1/2 gallon on a 4 gallon tank yields a 12.5% calculation error.

If you spread this calculation out over 10 tanks.... you will reduce your calculation error down to less than 1%.

Another big factor is driving conditions... Excess Wind Headwinds / tail winds, mountains, off road, city / highway and driving speeds... these all have a very measurable impact on MPG.
pizzy
Posts: 475
Joined: Tue Jun 02, 2020 6:59 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Leesbro63 wrote: Thu Nov 18, 2021 4:02 pm
mrc wrote: Thu Nov 18, 2021 3:36 pm Use the same pump. Fill to the first click, then more slowly until the second click. Stop. Repeat over several tank fulls. Close enough. If you keep a long and to this for a season you'll be as close as practical.

If you fill to the brim, besides mucking up the pollution control, you could lose fuel to expansion overflow with temperature increase.
This is good. I like this. Thank you. I'll do this. It reduces the risk of significant underfilling by a click that is premature, but doesn't go to the extreme of topping off. I might even go for the 3rd click, which will be soon after the 2nd click unless the 2nd click is also premature. This makes sense.
Out of curiosity, what good is this info?
Topic Author
Leesbro63
Posts: 7429
Joined: Mon Nov 08, 2010 4:36 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

pizzy wrote: Thu Nov 18, 2021 8:39 pm
Leesbro63 wrote: Thu Nov 18, 2021 4:02 pm
mrc wrote: Thu Nov 18, 2021 3:36 pm Use the same pump. Fill to the first click, then more slowly until the second click. Stop. Repeat over several tank fulls. Close enough. If you keep a long and to this for a season you'll be as close as practical.

If you fill to the brim, besides mucking up the pollution control, you could lose fuel to expansion overflow with temperature increase.
This is good. I like this. Thank you. I'll do this. It reduces the risk of significant underfilling by a click that is premature, but doesn't go to the extreme of topping off. I might even go for the 3rd click, which will be soon after the 2nd click unless the 2nd click is also premature. This makes sense.
Out of curiosity, what good is this info?
Helps get a more accurate number.
toomanysidehustles
Posts: 125
Joined: Tue Oct 06, 2020 10:09 am

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Leesbro63 wrote: Thu Nov 18, 2021 3:27 pm I'm wondering how to get an exact MPG reading. It seems that the only way to be sure that the gas tank is filled to the exact same point is to fill it to the brim where you can actually see the gasoline in the tube, just to the point before it dribbles out onto the paint. But I read this hurts the carbon cannister...yet I've never actually known anyone or even known anyone who has known anyone with this problem. How do you get accurate MPG readings?
Audi A6 Allroad AMAZES In Our 70 MPH Highway MPG Test

bestisfree
Posts: 89
Joined: Thu Jan 07, 2016 1:30 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Padlin wrote: Thu Nov 18, 2021 4:24 pm Just be careful to not overfill on one of those extra clicks.

From Gasbuddy.com among many others.
"Your engine is made up of a complex, closed-circuit system that traps and burns the fumes from your gas tank. When you overfill your regular or diesel fuel tank too frequently, the EVAP system that is designed to process excess gasoline vapor (not the overflowing liquid) can be compromised, affecting your car’s performance and resulting in a check engine light, as well as a costly repair bill."
This.

Please don't overfill. You may be doing more harm than getting precision.
BolderBoy
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

BatBuckeye wrote: Thu Nov 18, 2021 8:16 pmIt reminds me of a movie, the name escapes me, a neighbor of the main character obsessed over his MPG. The main character and his daughter, I believe, would at various and sundry times secretly add gas to the car. Maybe it was a VW bug. Where the neighbor just raved about his incredible mileage. Then there was one time when they siphoned gas out! His MPG seemed to plummet and the poor guy was on the verge of a breakdown.
This is a true story. In the late 1960s I entered the USAF and became a medic. At my permanent duty station we had an insufferable, young sergeant who used to brag about his new VW bug's gas mileage so we would sneak out and add a gallon to his tank here and there. His professed mpg went up past 100 and we implored him to think rationally. He continued to be a boor about it so we started siphoning gas off until he couldn't get out of the hospital parking lot without running out of gas.

Then we told him what we'd been doing, all had a good laugh and he said no more.
"Never underestimate one's capacity to overestimate one's abilities" - The Dunning-Kruger Effect
Metsfan91
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

Point wrote: Thu Nov 18, 2021 3:34 pm Reset the trip odometer on each fill up and you’ll
Be close enough.
This is what I do. It works for me. To get MPG, I simply divide the reading on trip odometer by the number of gallons I just bought. I buy gas at costco. I stop filling when pump shuts off automatically. I assume it is same type of gas every time.
"Know what you own, and know why you own it." — Peter Lynch
pizzy
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

Leesbro63 wrote: Thu Nov 18, 2021 8:47 pm
pizzy wrote: Thu Nov 18, 2021 8:39 pm
Leesbro63 wrote: Thu Nov 18, 2021 4:02 pm
mrc wrote: Thu Nov 18, 2021 3:36 pm Use the same pump. Fill to the first click, then more slowly until the second click. Stop. Repeat over several tank fulls. Close enough. If you keep a long and to this for a season you'll be as close as practical.

If you fill to the brim, besides mucking up the pollution control, you could lose fuel to expansion overflow with temperature increase.
This is good. I like this. Thank you. I'll do this. It reduces the risk of significant underfilling by a click that is premature, but doesn't go to the extreme of topping off. I might even go for the 3rd click, which will be soon after the 2nd click unless the 2nd click is also premature. This makes sense.
Out of curiosity, what good is this info?
Helps get a more accurate number.
For what purpose?
Metsfan91
Posts: 376
Joined: Sat Jan 11, 2020 12:33 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

pizzy wrote: Thu Nov 18, 2021 10:44 pm
Leesbro63 wrote: Thu Nov 18, 2021 8:47 pm
pizzy wrote: Thu Nov 18, 2021 8:39 pm
Leesbro63 wrote: Thu Nov 18, 2021 4:02 pm
mrc wrote: Thu Nov 18, 2021 3:36 pm Use the same pump. Fill to the first click, then more slowly until the second click. Stop. Repeat over several tank fulls. Close enough. If you keep a long and to this for a season you'll be as close as practical.

If you fill to the brim, besides mucking up the pollution control, you could lose fuel to expansion overflow with temperature increase.
This is good. I like this. Thank you. I'll do this. It reduces the risk of significant underfilling by a click that is premature, but doesn't go to the extreme of topping off. I might even go for the 3rd click, which will be soon after the 2nd click unless the 2nd click is also premature. This makes sense.
Out of curiosity, what good is this info?
Helps get a more accurate number.
For what purpose?
If nothing else, curiosity which knows not bound.
"Know what you own, and know why you own it." — Peter Lynch
cheese_breath
Posts: 10647
Joined: Wed Sep 14, 2011 7:08 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Does your part of the country change the gas formulation between summer and winter? That will really mess up your result.
The surest way to know the future is when it becomes the past.
Figaro
Posts: 55
Joined: Sun Mar 24, 2019 9:34 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

As someone who worked in the Petrol industry for many years - its never wise to top off your vehicle, especially in the way that OP suggests. Static electricity (say from your clothes or even your cell phone) could easily ignite the fumes from the gasoline.

MPG is all determined by how you drive. So if you have a lead foot on acceleration and brake instead of coasting to a stop - your MPG would be lower. Get enough of an avg by the on-board computer.

There's never a need to fill it all the way to the top - stop on the first click (unless you're trying to run away from Zombies).
tetractys
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Location: Along the Salish Sea

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Leesbro63 wrote: Thu Nov 18, 2021 3:27 pm I'm wondering how to get an exact MPG reading. It seems that the only way to be sure that the gas tank is filled to the exact same point is to fill it to the brim where you can actually see the gasoline in the tube, just to the point before it dribbles out onto the paint. But I read this hurts the carbon cannister...yet I've never actually known anyone or even known anyone who has known anyone with this problem. How do you get accurate MPG readings?
There’s absolutely no reason to fill the tank to the exact amount each time. As long as you record every time you fill up, your odometer reading and pump receipt supply all necessary data.

There will always be variations to gal./mile, depending on type of driving (city, highway) weather (winter, summer), tuneup interval, tires, etc.
cheese_breath
Posts: 10647
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### Re: Filling Gasoline Tank To The Brim To Compute MPG

Figaro wrote: Thu Nov 18, 2021 11:50 pm ... There's never a need to fill it all the way to the top - stop on the first click (unless you're trying to run away from Zombies).
If the zombies are real close, don't wait for the click. Just pump enough to outrun them. MPG be da__ed.
The surest way to know the future is when it becomes the past.
RobLyons
Posts: 1213
Joined: Tue Oct 31, 2017 12:55 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Perform a normal fill up, reset your trip odometer. After driving, return to the same gas pump and fill up again. Calculate.

Or do as I do and just look at the MPG readout on your dash 56.5MPG over the car's 50k miles !
"Great parenting sets the foundation for a better world"
rh00p
Posts: 151
Joined: Fri Jan 11, 2019 1:21 am

### Re: Filling Gasoline Tank To The Brim To Compute MPG

I didn't read much off this thread but as an FYI, topping up your tank can flood your EVAP canister. Driving on an empty tank prevents the fuel from cooling and lubricating the pump. Lastly, that fuel mileage monitor on your dash is for reference only, since ambient air temp and altitude affects the fuel's density.
Preparing for the worst. Hoping for the best.
bacon4retirement
Posts: 85
Joined: Sun Apr 19, 2020 9:59 pm

### Re: Filling Gasoline Tank To The Brim To Compute MPG

Also keep in mind that measuring the miles portion of MPG is also challenging. Typical odometers can be off by several percent. As tires wear over their lifetime, the reported mileage changes by a few percent due to the diameter of the tire shrinking. An example calculation is at https://www.busboss.com/info-tire-size.