Isn't wheel spin common (if not pronounced) when you're testing launch times?

I'd say a good accelerometer is the most precise (after actual light gates, of course), something off the speedo might be acceptable, but I really doubt the GPS is accurate enough for more than just fun. It's really not designed for high-speed, high-precision positioning. I imagine that there would be a lot of interpolation in the receiver itself a lot of the time.

Alex