A tachometer measures the speed of the engine. RPM stands for revolutions per minute and this is unit of measure the tachometer typically uses. A tachometer uses and electronic pule to determine how many times the ignition fires per minute. A speedometer measure how much distance is covered in a time span. MPH stands for miles per hour and KPH stands for kilometers per hour. A speedometer will measure distance by using a mechanical method, or an electronic method to determine the wheel revolutions. An electronic speedometer will use a pick-up on the wheel or inside the transmission to determine the wheel revolutions. A mechanical speedometer will use gears and a cable to accomplish the same job.