Speeding Up The Early Jeeps - Old School

I'm not sure of the year, but my guess is in 1958 or maybe a bit before, two die-hard Jeepers by the names of Clarence Shook of Rancho Jeep Supply and Basil Smith from ROCK-ETT Products, which later became Smittybilt, got together and built an adapter to mount a Studebaker-style Borg Warner overdrive to the output end of the Spicer 18 transfer case. While it could only be used in two-wheel drive, it lowered the overall rpm by 27 percent. This, in reality, changed those 5.38s to 4.10s and made highway travel not only quieter but faster. I found ways to modify the governor and solenoid so that I could shift into overdrive between each gear, turning the T-90 into a kind of six-speed transmission. For some reason or another I had lots of problems with the Studebaker overdrive and lost several planetary drives. The big drawback of this overdrive unit was the super-short driveshaft that resulted. Plus, you had to be sure to take it out of overdrive when you shifted into four-wheel drive or Reverse. The shaft and adapter sold for about $70, and the complete kit with a rebuilt overdrive went for $150 when they first came out.

Some time around 1962, Warn, the hub and winch people, came up with a compact design for an overdrive that bolted to the PTO output of the model 18 transfer case. Actually, I believe it was originally developed by a couple of brothers, Chet and Norm Thompson, who sold the idea to Belleview Manufacturing, which, in turn, was bought out by Warn. The first ones offered you a choice of either a 20 percent or a 30 percent overdrive ratio and could be used in four-wheel drive. It then later went to a 25 percent overdrive, which actually did a great job of splitting each gear in the tranny. The Warn unit was around until sales slowed, and in 1988 it was taken off the market. In 1991 Advance Adapters purchased the remaining inventory, blueprints, tooling, and renamed it the Saturn overdrive, which is still being made today.

There were a couple of imitators made by Husky and Dual-A-Matic that had limited success. And while they looked similar, parts were not interchangeable.

I remember spending $225 for the Warn overdrive in 1966. I sold the very same overdrive a couple of years ago for $800. So good was this overdrive that Jeep made it a factory option from 1964-'70. Even the military used it in its version of the FC-170 diesel-powered crew cab trucks.

For a while I ran both the Rancho and Warn overdrives in conjunction with each other. It made me feel like a trucker with all those gears to shift. Seems that I broke a few teeth off the Warn overdrive a couple time. Perhaps it had something to do with power-shifting it from direct to overdrive when drag racing.

The Saturn overdrive and rebuilt versions of the Warn are still popular with those who keep horsepower levels below 200 and retain the Spicer 18 transfer case.

Unfortunately, neither the Warn or Studebaker version of the Borg Warner overdrives would work with the new straight-through design of the Dana 20 transfer case that came out in 1972. But factory axle ratios had become more manageable by this time, and the four-speed T-98/T-18 was an option. Even automatic transmissions in the form of the super-strong GM TH-400 were available.

In the early 1980s a small machine shop produced a few prototypes of the Sierra overdrive for the Dana 20. I got involved in the development, and after some lubrication problems were solved, it proved to be a strong unit. Unfortunately, lack of development capital contributed to its demise.

Around 1969 Warn built an overdrive very similar to the Ranger overdrive offered today by Advance Adapters. Warn referred to it as an auxiliary two-speed transmission. This went between the T-18 transmission and the bellhousing. It was short-lived, and I only ever remember seeing one of them.