Re: speed sensor
What type of vehicle is this? If it's a wheeled vehicle, you could use a sensor to measure the RPM of a wheel (optically or magnetically) and use that to figure out how much distance has been covered in X amount of time.
Another possibility is to use a pitot tube and a pressure sensor, such as is used to measure speed in aircraft.
Probably one of the simplest ways, is to read out the velocity data from a GPS receiver. With complete units now selling for $25 or so in single quantities, this might be something worth looking into.