On the other hand...

I almost never run anything more than 5/8 -3/4 of a tank. I get much better gas mileage (2-4mpg) because of that and other steps I've taken to reduce weight in my truck. Those of you claiming better gas mileage when full... it's most likely because the level of the tank on almost all vehicles is inaccurate. so your first half on a 20 gallon tank should be 10 gallons. but generally they'll still read half till the tank is really down to 7-8 gallons left.