Chevrolet’s 1/2 ton and car overdrive 3 speed transmission was optional equipment installed on the assembly line during the 1950’s. The reduction of engine RPM’s in high gear resulted in less wear on the drive train as well as additional speed on level roads. Today, this is still important but of increased importance is better fuel economy.
The standard 3 speed transmission gives a 1 to 1 ratio in high gear. The overdrive is rated .7 to 1. The case and main gears are identical in both transmissions. The difference is in the rear extension tail. Here, the Borg-Warner gears electrically drop the RPM’s in the output shaft. GM’s wisdom created the 3 speed overdrive to be the same overall length as their standard transmission. This makes transmission exchanges very uncomplicated. There is no modification in the shift linkage rods or drive shaft.
With several basic tools a person can remove a standard 3 speed and add his overdrive in an hour! No problem if you don’t have the factory dash levers. Simply connect two insulated wires from the solenoid to a small dual position flip switch you add to the end of the shift lever. (It can be bought tat a local auto parts store and taped in place.) The driver can then shift in and out of overdrive using his thumb.
These overdrives were Chevrolet optional equipment from 1955 through the early 1960’s. Though they are becoming difficult to find, they do surface at swap meets, older salvage yards, and from owners totally modernizing their older vehicle. Find one and give your car or ½ ton a different personality!
Exploded View of GM 1950 Overdrive Transmission…PDF Click Here