I just got my fit and was looking over the manual when I saw it suggested keeping the tank under half full because less weight will help your mileage.

But would half a tank really make that much of a difference? Maybe the teeny bit over time would.

 

Every little bit helps. In my case since I top off the tank I'm carrying roughly 50 litres of fuel when full, which weighs roughly 80 pounds. According to Gas Mileage Tips - Driving More Efficiently every 100 lbs equals a 2 percent loss/gain in fuel economy. So if I were to reduce my fuel load to half (25 litres = 40 lbs) I should see a 1% gain in fuel economy (rounded for ease of calculation). In my case that would equal less than 0.5 MPG and is not worth the extra fuel stops that it would require. It is also not exact math either since half of my mileage would be spent with the same fuel load as the person that only fills to half, further negating any gains that would be seen.

For examples sake, a gain of 1 MPG over 20000 miles would result in a savings of 73 dollars based on the current gas prices in my area (4.30/Gallon). My time is better spent doing other things than spending time constantly filling up my car, especially for such small savings.

As always YMMV.

 

The problem with running it low is you MAY pick up water and gunk from the bottom of the tank.

 

Gas weighs 6 lb per gallon so the difference would be 30 lb; thats not enough to matter. You'd have to do a careful statistical experiment with carefully controlled conditions to see the difference.
As Loudbang said there would be more water drawn into the tank as the day cycles through low to jigh to low temps and that is definitely a problem especiaklly if your gas has ethanol added.
Its another example why engineers look askance at marketing.
cheers.

 

Haha what page was that on?

 

When I first started tracking my mileage, I had the same thought. So I did some fillups with more than half tanks left. It really didn't make much difference. I think that it depends on the driving that you do with light fillups. For instance, if all you do is city driving, your mileage will be less. If you do mostly highway driving, it will be more. Larger fillups give you a more accurate mileage over time because the average is more accurate. As someone else said, 30 lbs is like hauling a small dog around. Not very significant in the long run. If you want to see those fillups (they are early on in my history), you can go here:

https://www.fuelly.com/driver/teleburst/fit?fu=4583547