How does a bike speedometer work?

Answer:

The bike speedometer uses the measured circumference of a bicycle tire to calculate the bike's speed, using the number of tire revoluitions per minute as tallied by the sensor mounted ot the bike and the magnet mounted to the wheel. But with anemometer cups, there's a bunch of 'slip' involved -- the cups to not catch all the wind that goes by. So you can't just enter the anemometer's circumference into the speedometer. That's why calibration by some method is required. With our 2 anemometers, the offset number entered into the speedometer was 1320 (mm). If your unit is showing a higher speed than the truck or test anemometer, you increase the diameter that you enter into the computer. If your anemometer reads low, you decrease the diameter. A change of 5-10 mm makes a big difference! But after fiddling with it for a while, you should be able to get very close. If you get widely varying readings -- there is probably too much breeze for testing.

Answer

There is some kind of magetic field that's connects to the sensor on the wheel and the sensor of the forks    

Answer

when you go faster the speedometer tells you how fast you are going