It didn't seem to hurt my mileage, I drove 40 miles to my dealer and got 27 going and 27.5 coming back after the flash. One thing I did notice was when cold, It is a slug. The turbo lag is worse that ever. However, when it's fully warmed up, the turbo lag is completely gone. It always bugged the crap out of me before having to power brake it a bit before I wanted to take off to keep from geting hit by oncoming traffic, now it's instant. Now if they would finish my settlement review I would be a happy camper.