[G-CPTN]

Mounted in unit with the engine-gearbox assembly, but having an independent lubrication system from either of the other units, is a 48° bevel box. This incorporates helical reduction gearing, and it is this gearing which is changed to suit the requirements of any particular operator in respect of a specific final-drive ratio.
It is much cheaper to offer an alternative set of helical gears for the bevel box than to provide a variety of sets of crown wheels and pinions. In the Leyland final-drive arrangement there is one standard rear-axle ratio of 3.083 to 1, but a choice of final-drive ratios of 6.074, 5.334, 4.702 and 4.15 to 1.

From (and more at):- http://archive.commercialmotor.com/a...wn-to-business

Though that is 1958!