In the beginning, the Jeep was created with an off-the-shelf car transmission, designated as T-84 by its engineers. Tiny by any standards, and so were its capabilities. But serve it did through World War II.

With the advent of the civilian Jeep, the CJ-2A, a new stronger transmission was in order to replace the MB and GPW’s failure-prone T-84. The replacement, a T-90 three-speed, was compact and actually over-designed for the horsepower and torque output of the flathead four-cylinder engine. So good was this transmission that Jeep stuck with it for a record 20 something years. Without a doubt, a bigger variety of engines was adapted to this transmission than any other in automotive and industrial history. For instance, Baker forklifts used it and Studebaker used the T-90 trans up into the early 1950s, which made swapping in a “Stude” six or V-8 that much easier. It’s amazing that this little transmission designed for 60 hp would hold up well to V-8 power. Lloyd Novak started an adapter business with an adapter for this transmission.

When Jeep went to the V-6 Buick, they updated the design with helical gears and called it the T-86. While quieter in operation and smoother shifting, they soon found it wasn’t up to the V-6’s torque. Want to guess what the factory warranty fix was? Because the case dimensions were the same, the old T-90-style gears were a direct replacement. Neat thing about the T-90 was Jeep also used it in the utility wagon and pickups with their flathead-six. What made this great was that this version used a much longer input shaft that made building an engine adapter so much easier, as well as spacing the back of the then-popular Chevy V-8 with its rear-mounted distributor farther away from the firewall.

As good as the T-90 transmission was, it still required care if used behind a V-8. In reality, it was marginal. Much to my later regret, I once had a fuel-injected Chevy in front of a T-90. Needless to say, I got quite good at rebuilding it.

There were a few other transmissions used as well, but with 5.38 axle gears and 2.46:1 transfer cases, overall gearing was generally considered low enough. Jeep did offer the T-98 four-speed as an option on and off during the 1960s, and later, the updated version was called the T-18, but these were few and far between in the CJs. Contrary to information you may come across, there were some V-6s built with the T-98 four-speed on special order, and those that I have seen were all government vehicles. In the 1960s, the emphasis was on speed, not rockcrawling. Studebaker used the T-90, but in a column-shift version and there were some very early CJ-2As that had a column shift. One could use these cases and the then-popular Hurst shifters for faster gear changes instead of the cane and tower design. There were a few Ford passenger car three-speeds used with Hurst shifters, but these generally were one-off type of adapters. In 1960 or 1961, a Chrysler mechanic I knew built a few Chrysler automatic adapters for his friends.

 Warn introduced its overdrive in 1962, and it became an immediate must-have for Jeep owners with not only its overdrive feature but the ability to be used in four-wheel drive and to split gears.

The big change came when Brian Chuchua came out with his special adapter to use the Borg Warner T-10 four-speed. This was a performance trans used by Ford, GM, and Studebaker. It had an aluminum case, was fast-shifting, and, well, unfittingly high-geared. How does a 2.20 First gear sound? Yep, that’s what replaced my T-90. A step backward in gearing from the 2.9 First gear in the T-90, but oh so much stronger and fun to shift. But again remember this was the 1960s, when high-performance muscle cars were in and off-road racing was starting up – not rockcrawling. In the later part of 1969, a start-up company called Advance Tooling and Engineering made its first adapter for the Muncie passenger car four-speed to the Spicer 18 transfer case. You may better know this company now as Advance Adapters.

The factory 5.38 gears in the early Jeep differentials not only made for great acceleration but also some very high rpm at highway speeds. Spicer was offering a host of different ratios, such as 4.10, 4.27, 4.56 and 4.88, and those that could set up gears were trying different sets to gain more freeway-friendly engine speeds. However, they were not readily available in the aftermarket. It wasn’t uncommon to spend days in wrecking yards searching out vehicles that used Dana rearends for gears. I remember one set we put into a Jeep. The rear gears came from the back end of a Ford station wagon, and the front gears came from the rearend of a Studebaker Scotchman.

 The Rancho overdrive came out around 1958 in a joint development between Clarence Shook and Basil Smith. It consisted of a special adapter plate and a ’46-and-later Studebaker overdrive unit.

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.

 The Sierra overdrive was developed for the Dana 20 transfer case in the early 1980s.

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.

 Studebaker overdrive was the unit that was the basis for the Rancho Overdrive for Jeeps.

The Spicer 18 transfer case first appeared in the MBs and GPWs of World War II and was in production up to 1972 and came in 13 different versions over the years. Compact and quite strong, it is still being used by off-roaders today, and, in fact, there are several companies that specialize in custom gearing for them. One of the drawbacks of the intermediate gear design is that it rode on a shaft supported by a series of small needle bearings. This shaft never received sufficient lubrication, and both it and the bearings would wear and become quite noisy. It’s also thought by some that this shaft would actually flex under load, contributing to the wear and noise. Spicer, in an effort to solve the problem, increased the size from 31/44-inch (as used in the MB versions) up to 111/48-inch in the early civilians and finally ended production with 111/44 inch shaft. I believe it was Archer Brothers Jeep in Hayward, California, that came up with a way to eliminate this shaft and troublesome needle bearings. The company machined the ends of the intermediate gear to accept tapered Timken bearings. However, it was difficult to always get the correct bearing adjustment. Now it seems that with superior lubricants, the way to go is with the original design.

Believe it or not, there actually was a three-speed version of Spicer 18 built back in the 1960s. Thing is, it wasn’t built by Spicer. It was made from plate steel that was welded together and then heat-treated to relieve the stress before being machined. It had to be about 1962 or 1963, and I was for some reason or another in Buschert’s Machine Shop in Hemet, California. Harry Buschert was showing a buddy and I this prototype that he had developed. I don’t think that there were over a half dozen ever made, and each one was a bit different. I wouldn’t swear to it, but I believe it had a lower low range than the normal 2.46 gearing, a 1:1 high range, and an overdrive range. Harry was on the right track and way ahead of his time when he built these. Little could he imagine that some day a three-speed and a four-speed transfer case would be in production.

Today we have a wide range of transmissions – from standards to automatics – available, including six-speeds with built-in overdrives. Plus, we have adapters available to put just about any transmission to any transfer case your mind can imagine, as well as separate overdrive units and gearboxes that fit either in front of or in between the trans and transfer case. The old standbys – the Spicer 18s, Dana 20s, and Dana 300s – are being modified with lower gears and stronger parts to survive four times the horsepower and torque they were originally designed for. Transfer cases that were originally designed for much larger vehicles, not even carrying the Jeep nameplate, are being successfully used. Best of all, there are at least three custom-built transfer cases that can withstand all the abuse you can deliver. Sure beats the heck out of rebuilding your T-90 several times a year and new bearings for your Spicer 18 when you could no longer stand the gear whine.