I've used this:
GTech-Pro Model RR
In fact, here's some data that I posted from a track session:
Big Willow with SpeedVentures, Feb. 8 2009
It's a nice little unit, but there were three problems I had with it:
1) The mounting bracket is a bit flimsy... the ball and socket came apart and had to be glued back together.
2) The LCD display is impossible to read with polarized sunglasses.
3) The RPM data was sometimes spotty... i.e. dropouts in the data
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