For the purposes of this Act the weight unladen of a vehicle shall be taken to be the weight of the vehicle inclusive of the body and all parts (the heavier being taken where alternative bodies or parts are used) which are necessary to or ordinarily used with the vehicle when working on a road, but exclusive of the weight of water, fuel or accumulators used for the purpose of the supply of power for the propulsion of the vehicle, and of loose tools and loose equipment.