Naw - its so hard to see exactly when you hit 60 mph when you are driving, then much less hit the stop watch. Plus having someone else on board with you to do it will only slow you down...
Anyone with an iphone can download the accelerometer appplcation and just to it right from their phone. It automatically senses movement starts the clock, then hits the stopwatch for you when you hit the desired MPH....
But alas i have a blackberry...
maybe ill get this...
http://www.savinglots.com/lotprod.asp?item=MATRIX