Abstract: A system for regulating the maximum speed of a vehicle (10) having wheels (11, 12) fitted with tyres and driven from an engine (13) via a driveline (14, 15, 16) with a total overall gear ratio, the system including a means such as a GPS or radar based system for determining the actual vehicle speed of the vehicle without using a calculation based on engine speed. A control unit (18) is also provided which adjusts the engine speed and/or overall driveline gear ratio so that the maximum speed which the vehicle can attain remains substantially constant at a predetermined value irrespective of the diameter of the tyres fitted to the wheels.