Algodoo

A lot easier and more durable than a mechanical speedometer

(unless your method is better than mine: attach spring (with speed data scripted to spring length) to a gearified bar with a flat bottom resting on a bar, and then have the geared side spin a gear which is fixed to the needle. if you have any better idea let me know, please!)

RPM is orange and speed is purple.

(speed is the overall speed of the probe not the angular velocity of your driven wheels because you can only use 1 probe per scene, IE one for the engine and one for the driven wheels (in this scene I don't have multiple gear ratios so there isn't much difference but if you put this on a car with a gearbox it will be distinguishable!) )

Credit to Kilinich for the K-probe:)