I couldn't tell you the formula to calculate how to translate this speed into a specifc shift of pitch though.
Heh, I was told this formula at one point- I bet CS_TBL knows! cuz i can't for the life of me remember.
it would be interesting to see it.
Basically sound has a wavelength- the car standing still will emit sound with nothing added to the wavelength, if the car is moving forward the (velocityofcar-velocityoflistener) is added to the wavelength-
Bleh, it's in my head, I just can't describe it sensibly.