The speed and accleration of a car travelling in reverse and in the normal direction is the same. Therefore the measurement of its speed is still done using the metric system or via miles per hour (mph).
The measurement of the speed of a car in reverse direction is kilometer per hour or mile per hour.
The speed is a scalar quantity which means it gives only the quantity but not direction. The speed of a vehicle is generally measured with kilometer per hour or mile per hour.
The speed gives the measurement of the distance traveled (kilometer/ mile) by the vehicle in one hour. Since, it defines only quantity, in reverse or in normal direction, the speed is measured in kmph or mph with the help of speedometer.
If the velocity is measured which is a vector quantity. This indicates the speed and the car and the direction it travels. When the vehicle travels in reverse direction, the velocity is indicated with the ‘negative sign’.
