1954/55 Advance-Design Chevrolet trucks had an optional 4 speed Hydra-Matic transmission available on all 1/2, 3/4, and 1 ton models. The larger Chevrolet trucks did not have the Hydra-Matic option in 54/55.
Larger GMC New Design trucks (right up to the 950 series - 8 ton trucks?) had Hydra-Matic options by late 1955 but I am not sure if they were available in the Z-series (54/55) GMC trucks.
One of the late 1955 Hydra-Matic options was a Twin Hydra-Matic transmission ( . . . two four-speed transmissions mounted one above the other.) that had 7 forward gears. The Twin Hydra-Matic had optional 2-speed or 3-speed reduction units, resulting in either 14 or 21 forward gears.
Tim
