One thing we use on drag cars for the data logger is a rare-earth magnet attached to the tailshaft, set up a Hall effect sensor to read the magnet, and work out the road speed from the gear ratios (differential) and the tyre diameter.
2 things to bear in mind - The shaft will move up and down if the car has suspension fitted, and there may be tyre spin.
Second option is to attach the magnet to the front wheel - remembering that this may become airborne at times! :twisted: