Jeep Gearboxes and Their Ratios

How many of you actually know what model of manual transmission is clinging to your Jeep's underside? What about its ratios? An informal survey of Jeepers around the office wasn't very impressive.

By knowing which transmission you have and its ratios, you can do such things as calculate your rpm in various gears, have the ratios of other trannies at your disposal if you're considering a swap, and gain familiarity with which trannies came in other vehicles for easy spotting at the junkyard. But probably the biggest benefit is that you can look up your own First gear ratio and calculate the crawl potential of your rig for rockclawing and low-speed trail use.

Words of warning, though: First, ratios are usually changed by swapping in a transmission from another vehicle, often from a vehicle other than a Jeep. Why? It's actually fairly difficult to swap trannies around in the Jeep family--Jeep used so many different engine and transmission combos that simple interchangeability is nearly nonexistent unless you know Jeep like the back of your hand. Second, just because two transmissions have the same model name doesn't necessarily mean they're identical and will be a lateral swap. They could have slightly different gear ratios, input/output shaft designs, or shifter arrangements, such as the T-15 in a CJ and the T-15 in a Wagoneer. So if you're thinking of bolting in a new trans, talk to a professional shop about proper selection and installation. Thus, the primary benefit of this article is to tell you what tranny your Jeep has and its gear ratios.

We owe a debt of gratitude and our firstborn to Advance Adapters, The Gear Shop and Motive Gear for their exhaustive research and help with this story.