Well, he *does* have that blue hotrod in the Sonic & Sega All-Stars Racing game. Supposedly, he has a car because, if he just ran like he normally does, it wouldn't be fair to everyone else. Or something.
I dunno, if they guy can't even run fast enough to beat Mario in the 2008 Summer Olympics game, then the car might not be a bad idea after all! Plus, he gets to hang out with Flo, so, you know... ;-)