you'd use some sound extension that can change the pitch of a sound effect
"If Darl McBride was in charge, he'd probably make marriage unconstitutional too, since clearly it de-emphasizes the commercial nature of normal human interaction, and probably is a major impediment to the commercial growth of prostitution."
-- Linus Torvalds, December 5th 2003.
(Darl McBride is CEO of The SCO Group)
this place sucks but don't tell anyone, it's our little secret, ok?
I wished that existed or, if it exists, that I could find it.
Something else you could try is to have several car sounds and compare the speed of the car to play one of them (even though it wouldn't sound as good as changing the frequency).