The GM
Hydra-Matic transmission was first used in the 1940 Oldsmobile (that were available for purchase in late 1939).
These heavy cast-iron transmission were used ("battle-tested") in tanks and other military vehicles in WWII. By 1954/55 (I think), GMC sold a version of the Hydra-Matic in heavy trucks.
In 1955, there was a GMC heavy-truck 8-speed Hydra-Matic (7 forward and 1 reverse) that had two stacked gear trains (resulting with as much as a .70 overdrive). An additional 2-speed or 3-speed reduction-gearbox was attached directly to the rear of that seven speed Twin Hydra-Matic transmission (giving 14 or 21 forward gears). The lowest range of the 3-speed reduction gearbox was called the "creeper" range by GMC.
Tim
