It measures the rate of travel. IE. the speed the car is traveling at. The speedometer of a car measures speed. It is measured in miles per hour or kilometres per hour (mph or kph). The speedometer of a car measure the instantaneous speed.
In a car, the speedometer measured Instantaneous speed. This comes from the assumption that the car does not slow down and speed up fast enough for the speedometer to be able to give an average of the changing speed.
An automobile speedometer measures speed. That is why it is called a speedometer rather than a velocimeter. It tells you how fast the car is moving, not in what direction it is moving, so speed, not velocity.