1956 was the first year for an automatic transmission to be used in the 1-1/2 ton and up Chevrolet trucks.
The 4000 Series trucks had the optional 4-speed Hydramatic.
The 5000 Series and up had the optional 6-speed Powermatic that was made by Allison.
Both of these transmissions could be connected to either a 6 or 8 cylinder motor.
Mike B
