This reminds me of an product I used many years ago, I think it was called a Vericom VC-200. Basically a performance computer for your car that would calculate 0-60 time, 1/4 mile time/speed, braking distances, etc. It was based on an accelerometer of some sort.

I would think that GPS might have too much latency (even with corrections) and inaccuracies. Maybe a slightly better approach would be to use an accelerometer interfaced to the Empeg... The sensors are not very expensive, and produce an analog output. Maybe a simple voltage to frequency converter going to the microphone input? I don't know enough about programming (especially in Linux) to try it myself. The only downside to the accelerometer is you have run on level ground, otherwise going up or down hill will create false readings.

Now that I think about it, another approach could be just hooking up the speedometer signal to the microphone input (possibly with a simple level converter). This would be extremely accurate real-time speed/distance data (unless the car loses traction). A simple calibration factor for each unique vehicle would have to calculated/measured, but that's pretty easy...

Damn, if I only knew more about Linux programming...

Cheers,
Phil