Buzz:
What do you expect to learn from a GPS speedO? That is, why not use the one that is standard on your bike? If you're building the bike and it doesn't have one -- okay, that's easy enough to explain. But a GPS has some lag time from when it measures the speed to when it displays it -- maybe a second, maybe a few seconds. If that lag is okay -- then no big deal. Since you say you're building the bike for LSR -- you won't have to worry about times when you have no clear sky view (and therefore the GPS won't work).
Most any of the hand-held GPS units will have a real-time speed display and lots of other features, too.