A motor vehicle with power take-off apparatus is equipped with an arrangement for automatically controlling the rate of supply of fuel to an internal combustion engine which drives the power take-off output shaft. This arrangement has a sensor which produces pulses at a repetition speed proportional to the speed of rotation of the power take-off output shaft, and circuitry which provides an output signal at one binary level if the sensed speed of rotation rises higher than a first reference speed on increasing from zero, and at the other binary level if the sensed speed of rotation falls below a second, lower reference speed on decreasing from above the first reference rate. This binary signal is used to control the state, energized or de-energized, of a solenoid which in turn controls, directly or indirectly, a mechanical device which acts on the flow of fuel to the engine. The road speed of the vehicle can be limited automatically in the same way.