Why the sucky MPG?!

So many variables here. When I fill my tank, I fill to the brim most times. Then I go 400+ miles on a good day, but really only average 20-21mpg. If you fill until the pump clicks, you miss out on almost 2 gallons of extra capacity.
If you fill from empty to the brim you can put over 19 gallons in the tank. I've done it and I wasn't completely empty.

 

I completely forgot I wrote that. Must have been when I was in my Aerodynamics class lol. That was a tough class.

 

Not the greatest for the evap system but true. I always just fill my cars until the pump shuts off and I've never had any evap issues. I was actually surprised my 96 with 250k miles on it V6/auto 4x4 all stock is averaging 21mpg on the last 2 tanks. That's better than my old 05 double cab got with half the miles on it.

 

My family has always filled them completely full. We have a honda with 280k (bought new) and it's never been affected by it. I don't think it matters much as long as the vent design in the cap still functions correctly.

 

Everyone is entitled to do whatever they want with their own truck. The problem when you top off the tank completely you are often getting raw fuel in the vent tube which then can get into the expansion container, and then into the charcoal canister. On an older vehicle and perhaps some rare newer ones you would likely never notice an issue except for an occasional raw fuel smell. On OBD2 vehicles once the canister is saturated with fuel it's junk and you'll see a check engine light for sure. Considering how testy modern Toyota's seem to be with evap codes and the price of charcoal canisters I prefer to fill it only as far as Toyota intended.

 

oxygen sensor