Full tank vs. Partial fill ups for Fuel Economy - Fuel Economy, Hypermiling, EcoModding News and Forum

doomz78 · 06-22-2011, 01:30 PM

So would i be better off to put 6 gallons at a time versus dump the whole 20 gallons in my tank?

Weight difference
Would this really make a big difference in fuel economy? Or am i just splitting hairs now?

slowmover · 06-22-2011, 01:40 PM

IMO, yeah. Better to fill up weekly no matter how little used. At the end of, say, 10k miles the fuel used is still the fuel used, and there are far better ways to find another tenth (tires for the component "side" and driver skill for the other). I'd rather not ever have a dead air space in the fuel tank.

Kodak · 06-22-2011, 04:39 PM

As you know, the further you go into the tank, the more accurate average you will get, because little inconsistencies (fill variances, a few short trips, etc) will matter less.

However, I am weary of running the tank down all the way. I've heard about fuel pumps failing due to running on empty too much. Gasoline helps keep the fuel pump nice and cool. So no, I would probably not run on fumes on purpose.

Try to find a nice medium.

Phantom · 06-22-2011, 05:17 PM

Use at least 2/3s of the fuel before filing unless it will same you an extra stop on a longer trip. The reason why each time you make a detour just to stop and get gas you added distance to your trip and will add another open loop cycle using more fuel than if it was not restarted. If you turn right off the road go 150ft fill the tank and get back on the road by driving another 150ft in 17.5 stops you have added a mile of unneeded driving that could have driven you somewhere.

ConnClark · 06-22-2011, 05:23 PM

gas weighs about 6.25 lbs a gallon. So driving around with 6 gallons or less means your lugging 87.5 lbs (38.5 kg) less dead weight around than if you had a full tank.

To put that into perspective, it would require 14855 joules of energy to accelerate that weight to 62 mph (100 kph). In town it would require 3713 joules of energy every time you accelerate it to 31 mph (50 kph). These energy values are assuming a 100% efficient engine and no drag what so ever, so you'll probably end up expending more than 3 times this amount of energy.

Fat Charlie · 06-22-2011, 05:31 PM

I'd rather carry a few extra gallons than make a few extra stops.

ConnClark · 06-22-2011, 05:46 PM

Quote:

Originally Posted by Fat Charlie

I'd rather carry a few extra gallons than make a few extra stops.

For long trips I can see topping off the tank. For around town keeping the tank on the low side will pay off. I guess it is a matter of convenience vs saving money/fuel.

EDIT: It probably only takes 2 accelerations to 30 mph to equal the amount of energy to start your car.

California98Civic · 06-22-2011, 06:36 PM

If gas is 8lbs/USgallon and you are buying an extra 14 gallons more than you might (to fill the tank) then you are talking an extra 132 lbs. If every 100lbs can mean a 2% change in FE... there you go. Not tremendous but maybe not insignificant, depending on your car's base mpgs. I fill the tank all the way. I ran it down brutally low last time and saw pumping problems. Never again.

euromodder · 06-22-2011, 08:22 PM

Quote:

Originally Posted by doomz78

So would i be better off to put 6 gallons at a time versus dump the whole 20 gallons in my tank?

The error in filling up is always the same. The smaller the volume, the larger the error will be relative to the volume of fuel. For mpg accuracy, go for the least number of fill-ups.

Quote:

Weight difference. Would this really make a big difference in fuel economy?

In mixed driving, the effect will be marginal at best, and will get lost in the day-to-day variations.

Weight has most effect on stop-and-go (city) traffic.
But you'll have to look hard to spot the effect of losing say 50 lbs over only half a tank's distance.

I fill up when the fuel price is predicted to go up, and put off tanking when prices are predicted to drop.

Fat Charlie · 06-22-2011, 09:09 PM

ConnClark- And those differences are what makes up all of life. If I were driving in town I'd carry a lot less gas. Just about every time I turn the key I'm putting over 40 miles on the tank and some exits just don't have well placed gas stations. My commute times are so tight that I can't stop and get off the highway most days, so "just stopping for gas" isn't something that happens for me.