Filling Gasoline Tank To The Brim To Compute MPG

Questions on how we spend our money and our time - consumer goods and services, home and vehicle, leisure and recreational activities
User avatar
bertilak
Posts: 8934
Joined: Tue Aug 02, 2011 5:23 pm
Location: East of the Pecos, West of the Mississippi

Re: Filling Gasoline Tank To The Brim To Compute MPG

Post by bertilak »

tibbitts wrote: Thu Nov 18, 2021 3:45 pm 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.
Last time I got a new car, it had an MPG readout. I was always way off from what I carefully measured. It was over-optimistic. I even calculated a correction factor so, if I want4ed to, I could tell someone the car's actual MPG. It never came up, but did satisfy my compulsion for accuracy. It wasn't long before that got old!
May neither drought nor rain nor blizzard disturb the joy juice in your gizzard. -- Squire Omar Barker (aka S.O.B.), the Cowboy Poet
illumination
Posts: 1818
Joined: Tue Apr 02, 2019 6:13 pm

Re: Filling Gasoline Tank To The Brim To Compute MPG

Post by illumination »

I cringe when I see people topping off their tank, it really can ruin expensive emissions equipment.
I feel like I'm the only person that doesn't do this. Never have really understood the impulse.

I remember someone borrowing and filling up my car and did this to be nice, my garage reeked worse than any gas station for nearly a week.
User avatar
bertilak
Posts: 8934
Joined: Tue Aug 02, 2011 5:23 pm
Location: East of the Pecos, West of the Mississippi

Re: Filling Gasoline Tank To The Brim To Compute MPG

Post by bertilak »

illumination wrote: Wed Nov 24, 2021 3:45 pm I cringe when I see people topping off their tank, it really can ruin expensive emissions equipment.
I feel like I'm the only person that doesn't do this. Never have really understood the impulse.

I remember someone borrowing and filling up my car and did this to be nice, my garage reeked worse than any gas station for nearly a week.
I saw a sign on a gas pump: "Do not top up. Stop when it shuts off automatically." (Or words to that effect.) I always do that anyway. I'm too lazy to try to squeeze out the last drop and it turns out that's best anyway.
May neither drought nor rain nor blizzard disturb the joy juice in your gizzard. -- Squire Omar Barker (aka S.O.B.), the Cowboy Poet
User avatar
grabiner
Advisory Board
Posts: 30832
Joined: Tue Feb 20, 2007 11:58 pm
Location: Columbia, MD

Re: Filling Gasoline Tank To The Brim To Compute MPG

Post by grabiner »

bertilak wrote: Wed Nov 24, 2021 3:41 pm
tibbitts wrote: Thu Nov 18, 2021 3:45 pm 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.
Last time I got a new car, it had an MPG readout. I was always way off from what I carefully measured. It was over-optimistic. I even calculated a correction factor so, if I want4ed to, I could tell someone the car's actual MPG. It never came up, but did satisfy my compulsion for accuracy. It wasn't long before that got old!
I have seen the same thing. I reset my trip odometer at every fill-up to measure MPG. (My car has two, so that I can use a second one for trip distance.) The reported MPG is consistently 6% lower than the result of dividing the odometer by the gallons used. Since I drive a Prius, this is a noticeable difference, averaging 60 MPG versus 56. (The odometer reading is also about 0.5% below the actual mileage according to the GPS or road mileposts, although I think it is getting closer to accurate now that my tires are somewhat worn.)
Wiki David Grabiner
Makefile
Posts: 999
Joined: Fri Apr 22, 2016 11:03 pm

Re: Filling Gasoline Tank To The Brim To Compute MPG

Post by Makefile »

illumination wrote: Wed Nov 24, 2021 3:45 pm I cringe when I see people topping off their tank, it really can ruin expensive emissions equipment.
I feel like I'm the only person that doesn't do this. Never have really understood the impulse.

I remember someone borrowing and filling up my car and did this to be nice, my garage reeked worse than any gas station for nearly a week.
I think people used to do this back when paying cash and before "cash customers pay first" when they wanted to hit an even dollar amount. As gas gets more expensive but the pump speed remains unchanged, it gets more difficult too!

Also, can't the precise amount of ethanol in the fuel fluctuate from day to day? It says "contains up to 10% ethanol" but it could be less. That would itself have an impact on economy.
Post Reply