Better bicycle trip meters can be adjusted to the mm. You need to consider, however, that centrifugal force on a motorcyle tire increases it's diameter as you speed up.
I suspect that if you wanted to be that hinky about it however, you could mount it, drive a KNOWN distance and make adjustments, and be pretty accurate.
I looked into them, but decided to wait. Here's one that I thought was exceptional looking and cost effective:
http://www.branfordbike.com/computer/comp4.htmlmore:
http://www.houseofthud.com/bikecomputer.html