I have a couple projects about to begin on my 03 Spirit. First is a chain and sprocket upgrade. Going from a 42T to a 38T rear sprocket should help lower the rpm range just a bit on the freeway. This may or may not throw the mph off. (?)
Next is a jet upgrade along with a K&N (or similar) in the factory air box. This should give the bike a little more "umph" since the smaller rear sprocket will actually be slower on take off....not that I'm drag racing anyway. The mpg as of now is around 38 as I am running factory pipes with baffles cut out. The mileage may or may not change. Either way, this is an upgrade worth having!