If all else fails, he can pull the stock speedo and replace it. Down at the NAPA, they have two really nifty speedos. One uses gear drive, and the other uses a magnet and a sensor (similiar to a bike speedo). On either one, all you do to set it up properly is to measure out a set distance (I think 300'), press the set button, drive the distance, and stop. It sounds like it would be horribly inaccurate, right? Not really. Our speedometers in the tractors that run off of a radar system and are used to control all sorts of incredibly precise jobs (planting, spraying chemicals, etc.) use the same sort of calibration, and they are incredibly accurate.
Trying to come up with a witty line to end all of my posts...