If the speed sensor is outputting a speed-proportional pulse train, it may be as simple as polling the correct input fast enough and converting the vaules to a speed.

I wasn't paying attention to this thread, but then it dawned on me that the sensor issues are the same as with a rally computer. Earlier this yeat I installed a sense wire (tapped into the VSS wire going to speedo) and in the research for that it seemed like most everything out there now was a pulse, switch-to-ground. These aren't consistent b/w different cars, so our rally computer uses a base pulse rate and has a calibration feature (you input a car-specific factor based on indicated versus true mileage).

This page has a few examples of VSS inputs

A local gent who wrote his own rally calculator software also put this example of a very simple VSS-to-serial input on his page. Not sure if it would be of any help to anyone, but there it is.

With the rally computer we've been using, we've found the VSS to be a little fickle, so we are going to add a switched sensor like this and use VSS as a backup. Same switch-to-ground principle, same need to calibrate.

Hmmm with all these new input possibilities, will I need to get a tuner even though I won't use it for radio???