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...
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.