NV3550
Forward gearing: 4.01, 2.33, 1.39, 1.0, 0.78
Approximate length: 24.25-inch
Output shaft: 23-spline flush with back of case.
Found in: '00-'04 4.0L Jeep vehicles.
Aftermarket support: Swap bellhousings, adapters, and aftermarket shifters.
Notes: Some bellhousing configurations may place throwout bearing linkage on passenger side, necessitating hydraulic slave cylinder. Liberty models have shifter that points toward rear of vehicle, not straight up like TJ and XJ models, so fabrication may be necessary for shifter location. Up to task of handling moderate V-8 power and tires up to 37 inches.
T-90c
Forward gearing: 3.34, 1.85, 1.0
Approximate length: 8.12-inch
Output shaft: 6-spline.
Found in: '63-'71 four-cylinder Jeeps.
Aftermarket support: Replacement internal parts, remanufactured units, and engine and bellhousing adapters.
Notes: While standard T-90a gearing of 2.98, 1.66, 1.0 is close, the T-90c's deeper gearing gives the little L- or F-head a fighting chance. The T-90c case will still say T-90a. Shown is very early column-shift version which would definitely be a T-90a. Can pull top cover and count teeth for verification: T-90a has 18 teeth on input gear while T-90c has 16 teeth.
NV4500
Forward gearing: 5.61, 3.04, 1.67, 1.0, 0.73 ('95-'03 GM & Dodge); 6.34, 3.44, 1.71, 1.0, 0.73 ('93-'94 GM)
Approximate length: 12.34-inch (main case length)
Output shaft: 32-spline (GM) or 23-spline (Dodge).
Found in: '93 through mid-'00s GM and Dodge.
Aftermarket support: Bellhousings, performance shifters, internal replacement parts, and adapters.
Notes: Early GM versions becoming rare and used a 5.125-inch bellhousing index not as widely supported as the Dodge and '96-and-later GM 5.60-inch bellhousing index transmissions. Adapters available for 2WD and 4WD versions of transmission, but 4WD easier to mate to married T-case.
NSG-370
Forward gearing: 4.46, 2.61, 1.72, 1.25, 1.0, 0.84
Approximate length: 24-inch
Output shaft: 23-spline.
Found in: '05-and-later 4.0L, 3.8L Jeep vehicles.
Aftermarket support: Slim to none.
Notes: Overdrive not as good as most five-speeds, but overall tighter gear split from Second to Fifth will keep Jeep six-cylinder in its powerband and will help eliminate flat spots and lugging. Good choice for 4.0L guys, but leave the V-8s to the bigger, stronger transmissions. The crank sensor is in the wrong place for earlier fuel-injected Jeeps, and they have a 270-lb-ft input rating.
Mention (Honorable And Otherwise)
We left out a couple of transmissions. For starters, you'll notice there's no Ford C6, C4, or AOD automatics. It's simple-they don't always like to work at angles. When climbing, the pumps can become uncovered and the trannies just suck air while the engine revs and the vehicle goes nowhere. The hot swap for hard-core Ford wheelers is to use a JW Performance Transmissions (321/632-6205, www.racewithjw.com) Ultra Bell to mate a GM TH400 or TH350 in place of their Ford tranny.
Also, while we really like the AW4 auto transmission found in many 4.0L XJ, ZJ, and even WJ vehicles, its relatively complex wiring makes swapping somewhat problematic for the intermediate wheeler. We did stumble across one Web site outlining an AW4 swap into a '99 TJ that was very detailed and should prove helpful to anybody thinking about tackling a similar swap. Check it out in the drivetrain section of www.stu-offroad.com.