I want an overdrive in a 1940 sedan

March 1, 2012 | By Richard Prince


I have a 1940 Oldsmobile 90 Series, L40 4-door sedan with a straight eight flathead engine. I had the engine, which delivers 110 HP@3600 rpm, totally rebuilt by a professional. I would like to know if I change the Hydra-Matic transmission to a more modern transmission with overdrive, what late model transmission would fit? Could the rear end handle a more modern transmission? How would the linkage from the carburetor to the transmission work? Could the transmission perform with a 6-volt electrical system? Would the transmission work with the engine’s present vacuum system?


Most late model overdrive automatics will physically fit into your car without major structural changes. You will, however, need an adapter in order to attach a modern overdrive transmission to your 1940 Oldsmobile engine. Try contacting Bendtsen’s (transmissionadapters.com) or Speedway Motors (speedwaymotors.com) for an adapter that will work for you.

You will have a problem with any electronically controlled transmission because the controls for all of them are 12-volt. To overcome this you will need a power converter to convert the car’s 6-volt electrical system to 12-volts or you’ll need to convert the entire car to a 12-volt setup.

The performance and longevity of the original rear end in your car will not be significantly impacted if you swap your original transmission for a more modern unit.

A new transmission’s linkage to the carburetor and/or gear selector will need to be fabricated from scratch or adapted from an existing linkage.

If your replacement transmission requires a vacuum source, the original engine’s vacuum will work fine as long as the engine is in good operating condition.