The Sonic theme works great for my car since:
1) My car's color is Sonic Blue (obviously)
2) I am the store manager of a videogame store
So there's a double tie-in there.
5 years ago I got a bonus disc from work that had music from all of the Sonic games so I play that in my car at car shows as well. I've even joked that the yellow springs and red struts that my car has match Sonic's shoes.
But the best part is when you see a kid come up to your car at a car show, point at it, and have this BIG smile on their face when they see Sonic in the driver's seat.
There's even a guy on fj that likes my Sonic theme so he's decided to use "Shadow the Hedgehog" for his black Focus:
Oh, and as a theme for your car.....you could always do "Casper".