Ha, I had that idea this morning, but my girl said they'd probably just write me a ticket for not having a proper speedo. I'll ask one of them next time I see them outside.

 

You could just strap on a portable GPS receiver and eliminate all the guessing and calculating. When comparing my speed with a GPS and one of those stationary Radar signs that tell you how fast you're going, the GPS was right on the money. It's not so good for showing accelleration because it measures time over distance. That induces a lag in it's display. But for steady speeds it's great. It's amazing how far off our speedo's are on the upper end. I've seen 10 to 15 mph.