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

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:)

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.

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.

what skewbe said plus hills of any size...

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.

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.

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.

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.

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