On the other hand, it is considered safer to tow a 1.5 tonne car/trailer behind a 4.5 tonne motorhome, than to tow a 2.0 tonne caravan behind a 1.5 tonne car.A prime mover might have a GVM of 12 tonnes and a GCM of 62.5 tonnes as a B-Double. Explain how your theory works there, Schlitzy!