Maybe I'm the exception rather than the rule, but I drive with my Performance Shift on all the time, occasionally I will race some little rice burner or whoever comes along feeling froggy, I generally drive like a bat out of hell, and I'm getting 22mpg. Well, anywhere from 21.8 to 22.1. Are you burning at least 91 octane gas? I've found I get better mileage out of the higher octane gas, not to mention these cars need it to keep from knocking.