Originally Posted by goregasm
An explanation for the mpg issue is your car has more momentum with a full tank. Aerodynamics matter less when your car weighs more and you are traveling at a constant speed. Js.
Just because the car has more momentum doesn't mean the aerodynamics matter less, except relatively with acceleration and hills. The additional weight will compress the springs a little more, but I can't see the 50 to 75 lbs making that much of a difference.
On an ideal flat and level road there will be no difference once the vehicle is up to speed, except rolling resistance, which is a minor consideration. Someone here had some stats on that - maybe the Commander.
I have noticed the dash gauge is a little more optimistic when the tank is fuller though - are you reading off the dash gauge or actually miles/gallons at a fillup?