This is my first item i am submitting in instructables.This is an Awesome thing found by me.But I feel sorry that I didn't put the image.Leave it,See the step by step instructions and you will be amazed!Do not put this article in any other websites I beg you don't do this without my permission PLEASE!.
Now,to the materials
Voltmeter (one which has a needle(analog))
A motor (from a toy car).
some wires(It should be big enough for your bicycle)
The one am showing is connected to a multimeter!
First,you need to connect the wires to the motor.Now test the motor if it is working--Connect an LED to the motor and rotate the shaft.You should rotate in both directions because the LED wouldn't glow if current is passing throush the -ve terminal of an LED.after testing,you should remove the LEDs
Now connect the motor in such a way that when the motor is rotated the voltmeter reads +ve.
Now remove the dial of the voltmeter and place the calibrated dial.You can do this on a distance-marked track.the formula for calculating the speed is Speed=Distance/time
Now connect this calibrated speedometer to the motor and the item is ready!Now connect the motor to the wheels of a Bicycle.
How is it? Awesome? If it is good thanks.Here's the picture of an analog voltmeter.