The tune runs more timing and a leaner A/F than the factory tune, so I highly doubt its the tune. Our tune does not cater to gas mileage, but it certainly shouldnt get any worse than your factory tune by a mile per gallon or two. It can come down to the quality of gas you are getting, does it contain ethanol? When is the last time you replaced your fuel filter? There are many variables that come into play when you are talking gas mileage. I tell everyone this, the tune doesnt change. How long have you been running our tune? If the gas mileage all of a sudden gets worse, then something is going on mechanically with the car that needs to be addressed. Please feel free to call me directly at 574.594.9559 ext 3 if you would like to talk more about this. Hope this helps.