If this is a newer (95-Present) car you may want to think about interfacing with the onboard diagnostics computer via the OBDII standard. You will be able to compensate for all the variables mentioned above and make the device fairly accurate.

This company makes a chip that interfaces directly with the computer (check out the ELM327). You could tie your pic to IC and read the onboard computer directly:
http://www.elmelectronics.com/

I know your are looking to make your own device but here is some inspiration:
http://www.scanguage.com