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Overdrives, Underdrives and Gear Splitters

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Underdrive

An underdrive is a gear lower or below the normal gears used in driving a vehicle. It is most commonly referred to in the form of an external, additional gear box that provides extra gear reduction. A conventional underdrive typically will only be a 15 to 20 percent reduction, and is used either to give an additional gear below first gear, or as a gear splitter.

 

 

Extreme Underdrive

This is a specialized additional gear box with an extremely deep gear reduction. A conventional underdrive typically will only be a 15 to 20 percent reduction, and is used either to give an additional gear below first gear, or as a gear splitter. An Extreme Underdrive gives much lower reductions, such as 270% or 400% reduction. This really has no highway use. It is primarily useful in 4WD or farm applications where extreme low gears (way lower than factory 4WD low range) are desired. "Dual Transfer Case", Extreme Underdrive" and "Auxiliary Transmission" are all different names and different ways to achieve this general effect.

Gear Splitters

In some transmissions, the gears are so wide apart (example: you shift out of first at 3000RPM, goes into second at 1500) that it would be desirable to have another gear in between  gears. A gear splitter does this. They are primarily used in heavy towing applications, but other applications are at times useful. 

A gear splitter can be accomplished by adding an additional overdrive: Example; you are in second gear in the trans, and you engage the overdrive as well. You are now in what is commonly referred to as "Second-Over", which will be a gear above second, but still below third.

A gear splitter can also be accomplished by adding an additional underdrive gearbox: Example- 2nd gear in the trans, engage the underdrive, you are now in what is commonly referred to as "second-under": This gear is lower than second gear, but is not as low as first gear.  

"Ranger" Overdrive / Torque splitter

Saturn Overdrive/Splitter  (Dana 18 T-case applications)