The speedometer in a car does not measure the cars velocity because the velocity is

...because velocity is a vector quantity, which means that it includes both the magnitude and the direction of motion. 

The speedometer only measures the speed of the car and not its velocity. Speed is a scalar quantity, which means that it only indicates magnitude and not direction of the motion. Thus, the speedometer only shows the distance covered by the car with time (speed), but it does not indicate if the car is moving forward, backward, left or right.