The procedure I described, where you measure how many pulses per revolution of the wheel, and the distance travelled per revolution of the wheel, means you can now work out how many pulses are generated per kilometre. I have no idea how many that might be, but let's just say 1000 pulses per kilometre (one pulse per meter) to make the arithmetic easier.
When the car is travelling at 1 metre per second (1 m/s = 3.6kph) the pulses are one second (1000 ms) apart. Now, imagine the pulses are arriving 400 ms apart. The speed of the car is then: (1000/400) * 3.6 = 9kph. Thus by measuring the interval between pulses you can calculate the speed, using the principle just explained.
Let me emphasise something @oldvetteguy said: You need to find out how many pulses per revolution of the wheel you get, and you need to measure how far the car travels per revolution of the wheel.
Look at any 'C' tutorial to find out how to do basic maths like multiplication and division, then you will be able to get your Arduino speedometer working.
Have a go at it and post your code if you still need help.