I have some more bumperstickers but they are all in Flemmish, so the translation may not be that funny.

It's better to have a tiger in the tank then a donkey behind the wheel. (especially for those Porsche and Ferrari drivers)
You can laugh all you want, mine is payed! (usually on very small and cheap cars)
My yacht is in the garage. (idem dito)
Mijn Toyota is van plastiek/ My Toyota is from plastic(on the "My Toyota is fantastic!" logo)