They sound awesome, all of the cars exhaust sounds are almost dead on. The turbo's and superchargers sound sweet on this one, much better than Forza 2 where you really couldn't hear them. I just won an 07 Shelby GT 500 and the supercharger whine sounds really good, it's spot on.
Check out the finish to this drag race, I'm in the GT 500, we ran the exact time and somehow he edged me out by a millimeter!!!!!!
