Wis. Stat. § 347.41: Speed indicators.

No person shall operate on a highway any motor vehicle primarily designed for use upon a highway unless such motor vehicle is equipped with a speedometer which with reasonable accuracy registers the speed of the vehicle, except that motor trucks or truck tractors may be equipped with tachometers or any other devices that indicate speed and motor vehicles transported pursuant and in compliance with s. 341.47 (1) (b) and (c) may be equipped with a governor of speed of a type which restricts speed and which is set at a level equal to or below the limits of 55 miles per hour in lieu of a speedometer.