How to read a speedometer.

I want to arduinofy my electric bike. The first thing I wanna do is read the spedometer. The problem is I don’t know how it works.

My theory: The speedometer may actually be a voltmeter which gets the value form outside (How?). or the speedometer is intelligent and does the calculating the values and displaying them in analog (Again how?)

How do I know the speed of the bike. I think this is a good idea because it can act as parental control which lets parents know if thier kids have crossed “the speed limit”. This can either be achieved using gsm or just data logging (Will think about that later depending on budget). A red led can be set to on when a set speed has been crossed so the parents can check the logs.

But first how do I let the arduino interface with the speedometer or read the speed?

First, you have to dismantle the existing speed, and figure out how it works.

Yes, how is the speedometer hooked to the wheel or driveshaft?

A mechanical speedometer spins a magnet inside an aluminum cup. The magnet induces eddy currents in the cup and that applies some torque. The torque works against a spring so the needle turns until the spring tension matches the torque induced by the moving magnet.

An electronic speedometer usually has a pulse generator connected to one of the wheels. This is often one or more magnets passing a coil or Hall-effect sensor. The rate of the pulses is directly proportional to the rate the wheel is turning. The rest is software.

Whoa! That's a lot more than I can chew.

There is no way I can dismantle the speedometer because my dad wouldn't let me do it.

Can you tell me in plain words how to know what kind of speedometer my e-bike has (without taking it apart) and determine the speed. I'm sorry but I need more clarification.

Perhaps if you gave some information about the make and model of "e-bike" you have it would help. Online service diagrams would help more.

How do you expect to connect your Arduino to the bike without opening anything?

X-ray tomography?

Sorry about my it, but I thought all speedometers are the same. I will try to gather more info. Post photos etc.