You should be able to pick up a cheap bike speedometer at wal-mart or any store that has a decent bike section. you then drill a hole in the flywheel cover so the little sensor can be triggered by the magnet in the flywheel then it's just a matter of programing here's a website that tells you how
http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/
