ORIGINAL: JATO TO THE MAXX
Or you could just buy the Venom Speed Meter
[link]http://www.venom-group.com/itemdesc.asp?ic=VEN%2D0615[/link]
I have one of those, I used it on my R40. Nifty gadget to see how fast a car goes on a straight, but it unfortunately only adds unwanted weight on the track.
My only problems I have with this Speedo is getting the oll-out corect. Then, as the foam on the tires wear down, doing another roll-out to account for the reduced wheel's diameter.