Some tunes are legal. Some are not. By this thought, changing the final ratio would also be illegal because it alters the truck's OEM configuration that it was certified with. Almost all of the perceived improvements from tunes are from simply increasing the gain from the TPS. The little bit of actual hp gain is at max rpm WOT where we seldom if ever drive. I happen to like the lethargic feel of the loud pedal in my stock F250. I know it's there if I want to floor it but it encourages lighter throttle as it is. I like to try to go easy on the truck.