The Stovebolt Site Forums The Shop Area Driveline 55 1first series 3:90 rearend change

I would like more highway speed and want to change my stock rearend gearing. What is the best ratio to use, how difficult is it to perform? And what has to be done to calibrate the spedometer?

Since a 55.1 has an open driveshaft there are 3.38 gears for your housing, a big difference. A Borgwarner overdrive bolts right in or many go the T5 route. MUCH easier for either than the torque tube trucks. Cheapest is the 3.38 gears IF you can do the work yourself, if not it's about a wash in cost.

Adding to the above:


MPH = (RPM X Tire Diameter) / (Final Transmission Gear Ratio X Rear Axle Ratio X 336)

Desired cruising RPM (factors: best engine torque - best gas mileage - empirically to taste)
Tire diameter (in inches)
Final gear ratio of transmission (transmission input to transmission output)


For speedometer:
Change the cable drive gear in the transmission or purchase an external speed adapter.
Calibrating the speedometer gauge itself is best left to a professional gauge restorer.

I have never heard of the overdrive system. Is that in addition to the 3:38 gear? Or used with my stock see up?

The overdrive transmission is by Borg Warner, search that and R10 and/or R11 terms for plenty of articles. It is a 3 speed front trans with a planetary gear set behind that. It provides a 0.72 reduction when engaged.

This is a tough subject to get right because there are a lot of options on how to go about it.

I would have the rear axle gears checked for wear, or do it myself, before deciding to keep the original axle. I personally would keep the axle cause it fits the fender wells the best compared to an axle swap. Changing out the gear set can be expensive and requires an experienced mechanic to get the mesh correct.

These old trucks were designed for a time when 55 mph was about as fast as you could go on the roads back then. To get more speed you need to make sure your brakes are in good shape or update the front to disc (a matter of taste). The steering needs to be tight with minimal wandering. I mention this because white knuckle driving is not fun.

The 3 speed overdrive transmission was a factory option for Chevy cars and trucks from 1955 through the early 70's.
The first version which would be a bolt in for your truck, is a Muncie SM319. (1955-65) It uses the same basic transmission as your truck with a Borg Warner R10 overdrive unit that replaces the rear tail housing. It uses a cable to lock it in or out of overdrive. The electrical system consists of a solenoid, governor, relay, kick down switch, and a wire loom.

Chevy came late to overdrive's, Studebaker, Ford and other mfg's had them in common usage much earlier.
Fred