A cars speedometer measures the rate at which the wheels spin, which is correlated to (using an assumed diameter for the tires) a speed over the ground in miles per hour or kilometers per hour. The speedometer only gives direction as "forward" therefore does not truly represent velocity, which is a combination of speed and direction. A common feature of many speedometers (all that I have ever heard of) is an odometer, which measures total distance traveled, a good indication of the wear on the vehicle.