depends. with a car and flat bed I get no noticeable difference in mileage. Towing my boat 6-8 mpg depending on how many hills I go up. crossing the pass it drops to about 5.

I'm hoping that with more power I will have to work the engine less, which should = more mpg

C-Leigh Racing

Thanks for the input. It's good to here from another tow rig. Sounds like you are really hauling a load! I will definitely be doing some work to the heads.