Would we get better mileage runing 1/4 tank than always filling up?

[DarbyWalters]

Just wondering what the weight of the extra, in my case 15 gallons, fuel does to our mileage...? If diesel fuel is 7.206#/gallon...would the 108# savings benefit mpg much?

[zpiloto]

Originally Posted by DarbyWalters

Just wondering what the weight of the extra, in my case 15 gallons, fuel does to our mileage...? If diesel fuel is 7.206#/gallon...would the 108# savings benefit mpg much?

I don't think it quantifiable in terms of the equipment we have to measure. The EPA says to remove excess weight to increase FE but I don't think 100# on a 3000# will show up. The lighter the car the better I guess. It definately helps with acceleration. Might want to do a search. I did some playing around with this in a Mazda 626 which has a 15 gallon tank and did several tanks with 4 gallons. No noticeable increase if FE. Plus it was a hassel to fill every 2-3 days. YMMV

[trollbait]

I had noticed that I got better milage towards the end of a tank with the Prius, so I tried only filling it part way. It seemed to help, but my technique was also improving, tires wearing, etc. I may try it again with the Ranger.

A co-worker that moved up from the south pointed out that they couldn't partially the tank. The humidity could lead to rusting in the tank.

[skewbe]

It really depends how you drive. If you are good at staying off the brakes (including engine braking) it won't make much difference, you will simply glide about 1/30 farther than normal for a given car (and have to accelerate 1/30 more).

If you are wasting a lot of energy braking then you will get about 1/30 worse mpg with a full tank. Note the 1/30 is 100 lbs of fuel in a 3000 lb car. A smaller car/fuel tank will have a different ratio.

[2TonJellyBean]

what skewbe said plus hills of any size...

[Hateful]

I did see a gain carrying less gas,but after all the other measures taken since then, I didn't see the same loss when I filled all the way up again. It's cold now and I don't like getting out of the warm car to pump gas. Also the pump stops are just adding to low FE parking lot driving.

[Danronian]

Originally Posted by Hateful

Also the pump stops are just adding to low FE parking lot driving.

That's what I was thinking. Most times during rush hour there is also lines at the pumps where I live, so that would just make mores stop-go driving. I think on a car with a large tank, this issue of not filling up all the way could be helpful, but with a mere 10 gallon tank on my civic, I don't think there would be much difference.

[2TonJellyBean]

Just do the math on if the weight affects things...

6.2 pounds per gallon (premium is 6.35 PPG)

Added mass affects acceleration and climbing. As a heavy cyclist I can tell you for a fact, that mass and hills is bad. What you lose going up always exceeds what you gain going down, both in time and energy.

[palemelanesian]

Analyzing my commute since April (270 data points)yields these results:

Full tank: 54.5 mpg
3/4 tank: 54.7 mpg
1/2 tank: 54.7 mpg
1/4 tank: 54.5 mpg
empty: 53.8 mpg

I try to run the maximum per tank, and minimize the number of fillups.

[1993CivicVX]

Originally Posted by DarbyWalters

Just wondering what the weight of the extra, in my case 15 gallons, fuel does to our mileage...? If diesel fuel is 7.206#/gallon...would the 108# savings benefit mpg much?

No, but you might get worse due to increased vapor loss.