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Truck Transmissions - High Gear!

Joel Mollis | Writer
Posted November 1, 2000
Photographers: Randall Jachmann

Operation Overdrive: Modern four-Speed Overdrive Transmissions for three-Speed Trucks

The old adage "You can't have your cake and eat it too," doesn't always apply to performance-enhancing upgrades for our off-road vehicles. If the goal is the installation of a component that enhances fuel mileage as well as improves performance, then one of the most effective ways to achieve such results is with the installation of a modern four-speed Overdrive transmission in place of a three-speed slush box.

The enhanced economy provided by a four-speed Overdrive is basically the result of a tall transmission ratio (typically 20 percent higher than a three-speed's high gear), which puts the engine at the lower part of its powerband during highway and cruising speeds. Simply stated, when the engine is turning low rpm but still making enough power to propel the vehicle, fuel mileage is improved. How much of an improvement? You'll realize that is a complex matter involving your vehicle's aerodynamics, tire size, the engine's state of tune, and the final drive gear ratio in its differential(s).

Interestingly, many trucks that are factory-equipped with three-speed technology - both 4WD and 2WD - can be upgraded to four-speed performance quite easily. In many cases, the transmission swap is a direct bolt on, requiring few - if any - additional accessories and virtually no fabrication of custom parts. As to the actual selection of an overdrive, there are basically two ways to get your truck equipped with four or moreforward speeds: Swap out the three-speed and replace it with a four-speed transmission, or keep the three-speed in place and augment its selection of gears with an add-on overdrive/underdrive gearbox. There's something to be said for either method. You can even equip a four-speed overdrive transmission with an overdrive/underdrive gearbox for the ultimate performance automatic transmission, but that's another story.

What follows is an examination of the latest ways to get your 'wheeler into Overdrive, including transmission identification, overdrive theory, several new products to make the upgrade to Overdrive a more straightforward process, and an analysis of an add-on overdrive/underdrive gearbox for any automatic transmission application.

GM Automatic Transmission Identification GuideThe easiest way to identify a GM automatic transmission is through an examination of the shape of the pan gasket and by noting the number of fasteners that retain the fluid pan. The guide is also useful if you're shopping for a new overdrive transmission or looking for a core trans to rebuild. The arrow indicates the front of the transmission and the vehicle, which helps to further clarify the identity of any particular transmission.

General Motors trucks that are equipped with a three-speed automatic transmissions use one of the following models: the Turbo Hydro (TH) 350 or the TH400. Typically, the TH350 was used behind small-block engines; the TH400 was factory-installed behind Rat engines exclusively. Late-model GM trucks equipped with four-speed overdrive transmissions use either a TH700-R4 or a 4L60/4L60E (small-block applications) or a 4L80E (big-block engines).


JET Performance Products
Huntington Beach , CA
Gear Vendors
El Cajon, CA 92020
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