Soeedometers perform the most precise measurement you can get. When you fit a nonstandard tire you just introduce a constant scaling factor. That is not so bad.

Soeedometers perform the most precise measurement you can get. When you fit a nonstandard tire you just introduce a constant scaling factor. That is not so bad. GPS on the other hand is atrocious. Don't even try to get an accurate position in places like downtown Seattle. I'm regularly getting misplaces by several blocks because of occlusion and reflections from the buildings. You just can't compute an accurate speed from such a signal. Besides, GPS can only update once a second. And given the signal jitter you can't get a good speed reading from the last two fixes - when you are going in a straight line. When you are not going straight, the GPS based estimate must underestimate the distance you travelled and thus your speed. Using this toy speedometer thingy for driving is just dangerous and insane and illegal in so many ways that I wish the article could just be pulled from the web.