ORIGINAL: Nashcat
I saw some info on calculating speed using a sound recording of the engine as it passed by a microphone. You could record the sound on your computer and the program would calculate the speed by using the Doppler effect. I can't seem to find the info now that I have a use for it.
Nashcat
I didn't read through the entire article, but this sounds similar:
http://www.alphalink.com.au/~hughan/speed.htm