Anyone who has driven an early Jeep can tell you they are very fun little vehicles that will go pretty much anywhere. They can also tell you they are very underpowered and slow on modern roads. When my father owned my Jeep, he remedied the first problem by replacing the original 4-cylinder engine with a Chevrolet V8. He once told me the original engine didn’t have enough power to pull your hat off.
The second problem wasn’t just a matter of power; it was also a matter of gearing. Early Jeeps like mine came with 5.38:1 axle gears which makes it nearly impossible to do much more than 45 mph no matter how much power you have. Fortunately, the layout of these Jeeps’ drivetrain makes it surprisingly easy to add overdrive, and a couple of companies made kits to do it. The one dad used was made by a company called Dualmatic.
Jeeps of this era used what is called a “double offset” transfer case. Most transfer cases have the rear output in line with the input and the front output offset by some distance (usually somewhere around 9”) and lower than the transmission centerline. That is considered a “single offset.” When the rear output is also offset, that is a double offset.
When you remove the cover plate on the back of the Jeep transfer case, you are looking at the input gear for the transfer case, which is directly attached to the transmission output. The overdrive kits replace this gear with a planetary gearset and shift mechanism. Here are a couple pictures of mine from when I recently had to replace the old overdrive.
The last image I have here shows the overdrive unit on the bench, to give you a better idea of what it consists of.
This upgrade makes a world of difference for drivability and is actually a period-correct modification. Also, it’s just a really clever design, which is something I appreciate!
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