How to transfer speed on a rc car to mph??

I want to know how to find out how fast my rc is going,and I really want to know how to transfer the speed that is coming from the motor or motor to a specific speed,like how bikes are when you pedal,and some bikes say how fast your going.Like a dyno,how do I build something like a dyno for a small rc car ??

I need it to be exactly like a small dyno,I dont know the rpm's of it.

Do you know the RPM that the wheels are spinning at? If you have this number, you can measure the circumference of the wheels and with these two numbers you can calculate the MPH. For example:

Lets say your rc car has a max RPM of 2,400 and the wheels have a circumference of 10 inches.

10*2,400=24,000

That means that every minute the car travels 24,000 inches. Multiply by 60 to get the distance per hour:

24,000*60 = 1,440,000

So the car goes 1,440,00 inches an hour. Since there are 63,360 inches in a mile, divide by this number to get the MPH.

1,440,000/63,360=22.73

So in this example, the rc car goes 22.73 MPH.

Bikes count the revolutions of the wheel from a magnet and multiply by the circumference of the wheel.
So one way would be to get a tachometer, contact or non contact and multiply times the outside diameter of the tire times Pi. That will give you real inches per minute of the tire. If you want to make it scale miles per hour, you have to multiply times the scale of the vehicle - like 1:24 - and by 60 to go to hours and divide by 5280*12 to go from inches to miles.
http://www.reliabilitydirect.com/tachome...

Get a Radar gun makes it easy.