a dropping resistor will fix it, but u need the right value resistor. get a 10k potentimeter and start from there, adjust it, drive it, adjust it... drive behind someone you know that is doing a paticular speed, and adjust it til you get the speed correct. then take to potentimeter out, measure the resistance (ohms) of it and get a fixed value resistor at that value.
hope that is understandable... cheers