Abstract: Linkage is disclosed for adjusting the position of a diesel engine injection pump control rack in dependence on the position of the driver's accelerator pedal and also on a system for automatically maintaining the vehicle speed at a desired value. The injection pump rack is positioned by means of a pneumatic piston-cylinder assembly and connecting linkage. The pressure in the piston-cylinder assembly is adjusted by means of a driver-controlled valve in a supply line from a pressure source, and this valve is adjusted by the driver's accelerator pedal. A normally open valve is also connected in the pressure circuit and a normally closed exhaust valve is provided. When the automatic speed control system is in operation, the driver fully opens the driver-controlled valve (by fully depressing his foot pedal) and the automatic system adjusts the other valves to hold the speed at the desired value.