(1) The speed of twenty-five miles an hour shall be the ordinary speed limit for light motor vehicles when being used for the traction of another vehicle.
(2) The following speeds shall be the ordinary speed limits for heavy motor vehicles of which all the wheels are fitted with pneumatic tyres, that is to say: -
(a) in the case of any such vehicle which is a large public service vehicle -
(i) if it is fitted with a floor for the seating of passengers the whole or any part of which is vertically above the whole or any part of another such floor, the speed of twenty miles an hour, or
(ii) if it is not so fitted, the speed of thirty-five miles an hour;
(b) in the case of any such vehicle which is not a large public service vehicle -
(i) when it is used for the traction of a vehicle of which all or some of the wheels are not fitted with pneumatic tyres, the speed of ten miles an hour,
(ii) when it is used for the traction of a vehicle of which all the wheels are fitted with pneumatic