I've been trying out one of those $10.00 bicycle speedos. Before I set it all up, I was gps'd at a steady 36mph on the flats. This is on a 2-speed za50 magnum. Stock gearing 18x40.
I Measured the O.D. of my front tire @ 1800mm. Then set the speedo unit to that tire spec. Getting the magnetic sensor close enough to the receiver was tricky. I used a foam cube spacer, & connected it to my fork stabilzer bar. (see pic). I went on a test run, and sure enough....36.8 mph.
SO, Last night, I re-geared my rear sprocket, from a 40t to a 38t. Went out this morning for a test run, and top speed increased to 38.2mph, according to this speedo. It also has nice features like Max speed, Average speed, Miles traveled, Temp, & Time of day.
I will get gps'd again soon, but I think these things are pretty accurate if set up properly. Anyone else running these?