The spoiler will help on the order of a few tenths of a mpg on typcal highway driving. No impact at all on city driving. The faster you drive on the highway, the more improvement you will get (function of vehicle speed squared). You may not notice / it may seem insignificant for any one trip but over time, it will add up. The spoiler improves the aerodynamics of the vehicle. I would take it back to the dealer and have them install the spoiler - they are supposed to do it before you get the vehicle.