So, how much does a supercharger decrease gas mileage? Let's say I get 18 mpg just going down the highway. What would it decrease with a supercharger, assuming I drove the same and didn't put my foot into it?

'01 Rodeo Sport, 5 Speed Softie, Indy 4X rocker bars, OME 912's, Rancho 9000X's, Superwinch hubs, custom tube bumper with Warn 9000i , Flowmaster 50, TeraLow t-case gears, ARB front & rear lockers, TrXus MT's, Sway bar disconnects