An overdrive for a ’57 Chevy

Question:

I have a 1957 Chevrolet 210 with a 230 cid six-cylinder and a three-speed manual transmission that has been in my family since it was new. Except for a Pertronix ignition (and later this year, radial tires and a dual circuit brake system), I’ve kept it stock, and I won’t make any changes to this car that can’t be easily and totally undone.

That said, I’d like to swap the transmission for an overdrive unit.

I get about 14 mpg, and although I haven’t put a tachometer on it, it sounds like it’s spinning pretty fast at highway speeds. It has the stock 3.55:1 rear axle gear ratio, and although I believe 4.11:1 was the standard ratio for OD-equipped cars, I’m sure my engine (only about 60,000 miles) can handle the 3.55 with the OD.

I believe sometime in the early 1960s GM added a synchronizer to first gear, and I suspect these transmissions may be easier to find than a 1957 unit. What years will fit my car? Finding all of the associated linkages, solenoids, etc. seems to be a challenge with the 1957 units also. Will there be any problems with these parts from a later vehicle? Is there an overdrive unit I could use that replaces the existing transmission tail shaft?

Answer:

Chevrolet began offering the three speed overdrive transmission as an option in 1955 and continued to do so until the mid-1960s. The overdrive’s step-up sun and cluster gear assembly provide the overdrive function, enabling the driveshaft to spin faster than the engine.

It spins about 1/3 faster, which means that for a given highway speed the engine in an overdrive-equipped car turns about 1/3 fewer rpm than an equivalent car without overdrive.

The overdrive is a distinct unit that mounts in back of the three-speed transmission but the three-speed is different when coupled to an overdrive.

Besides a different tail housing, the transmission’s output shaft is longer, enabling it to insert into the overdrive.

The best way to execute the overdrive conversion you’re contemplating is by buying a complete overdrive setup, including the transmission, overdrive unit, solenoid, relay, wiring harness, and kick down switch. Even if some of the electrical components don’t work, as is often the case, you will at least have the parts to start with and in most cases they can be fixed. A system from any year it was offered will work in your car.