An underpowered TV usually will not get better MPG than a TV with a larger engine while towing. I have towed with a TV with a 350 engine 1/2 ton and towed with a TV with a 454 engine 3/4 ton. In both cases the mileage was in the 8.5 MPG range. I was towing the same TT in both cases. The 350 TV did get better solo MPG. Ben