Going down hill will allow an electric car to travel faster than it would on level ground. An electric motor has its greatest torque at the very bottom of its RPM range. An electric motor will never see its peak RPM in a loaded state for this reason. If you reduce the load (ie gravity pulling the car down a hill) the motor will be able to reach a higher RPM. You cannot use the maximum RPM of an unloaded electirc motor in your calculations for maximum speed in a vehicle, it simply doesn't have the torque to move the load at the higher RPMs.