Both posts above said your mileage will get worse. It will. The bottom line is your engine will be turning at higher rpms for the same speeds. In order for an engine to turn higher rpms, more fuel is required. Your gas mileage will suffer, but it won't matter. You are going to be so busy trying to peel the smile off your face, you won't even notice the difference in fuel mileage.
Take care,
~Chris