Abstract: A transport refrigeration system including a compressor, an evaporator, a condenser, an air mover assembly for the evaporator and condenser comprising fans and/or blowers, and a prime mover for the compressor having first and second selectable speeds. The speed of the air mover assembly is mechanically interlocked with the speed of the prime mover via a pulley driven speed changer, to eliminate the possibility of overspeeding of the air mover assembly. The speed changer changes the speed of the prime mover by a mechanical movement which cannot be made without tension in the pulley belts automatically changing the pulley ratio of the speed changer, to maintain a substantially constant speed for the air mover assembly, notwithstanding a change in prime mover speed.