I used to ride an inter-city bus to and from college that was probably built just after WW II. The tachometer served as the speedometer. It had a scale in MPH for each of the 5 forward gears in the transmission. This was a Flxible (yes, the spelling is correct) body on a GM chassis. I suppose you could do the same with a little calculation if you knew the ratio of each of these gears and calculted that against the final output to the wheels. You would have to take into account the diameter of the wheel.