also you could try to find a speedometer that police uses (you know that gun). aim that towards your car and there is a display allredy built on it. you get a perfect result and if you have a policeman sitting beside you you could even give someone ticket for speeding. alsoyou could mesure the speed by reading how fast houses/trees/cars pass you. it's all math. 5th wheel or a block of iron might be better, but i reccon that my ways are a bit cheaper (and easyer).