I purchased a Hypertech "chip" several years ago for one of my Mustang GTs and was happy with it so after reading reviews of several brands I decided to try it again. I haven't had it long enough to determine gas mileage gains but it doesn't appear to be detrimental (if I can keep my foot out of it). I especially like the fact I can eliminate the "skip shift" feature (under light to moderate acceleration the factory car computer forces a 1st gear to 4th upshift) that is so frustrating. I installed the Hypertech with a cold air intake so my gains in power may be due to both. I used to get a chirp going into 3rd, now can get the car sideways! Cold driveability has decreased but that could be a factor of the intake, and it smooths out when warmed up. I would probably buy this again.