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).
Further identification is as follows: TH350 transmissions used on '69-to-'91 GM 2WD and 4WD trucks are either a lock-up (TH350C) or a non-lock-up (TH350) style. Both the standard TH350 and the TH350C can be identified by the vacuum modulator, which is located on the passenger side toward the rear of the transmission. The TH350C also has a wiring harness that plugs into the transmission on the driver side at the case's mid point; the TH350 has no electronic controls. The TH400, used in trucks during model years '65 to '95, sports a vacuum modulator on the passenger side of the forward section of the case. The TH700-R4, used on trucks built during 1982 to 1993, uses a TV cable that's attached to the front side of the case on the passenger side. GM's 4L60E four-speed overdrive was installed '93-to-2000 trucks, although slightly different versions of the 4L60E were installed on trucks built after 1997. The 4L60E has a wiring harness and a connector, which is located on the case's passenger side near the center. The 4L80E was also manufactured in two versions, but are identical in outward appearances. Both the '91-to-2000 and the '98-to-'99 models of the 4L80E use a wiring harness and an electrical connector located on the passenger side of the case near the center.
GM Automatic Transmission Gear Ratio Chart
Take a GM TH700-R4 four-speed transmission, load it with upgrades, and you'll have a JET T-700. Highly modified for performance use, JET's T-700 is a direct replacement for the TH350, and features a Corvette servo shift piston, a 12,000-pound GVWR towing capacity, a performance valvebody, and additional clutches in the three-to-four pack. The T-700 will reliably handle as much as 450 lb-ft of torque, making it an ideal replacement transmission for small-block applications.
For big-block GM trucks, JET offers the 4L80E, which is beyond stout - it will handle 700 lb-ft of torque, and has a GVWR of 24,000 pounds. In fact, JET claims its modified 4L80E is as strong as a TH400, but with the added benefit of an Overdrive Fourth gear. The JET 4L80E four-speed features a 34-element sprag for strength, a performance-bred valvebody for positive and responsive shifts, heavy-duty clutches and bands, and a lock-up torque converter feature. When installing a JET 4L80E transmission in place of a TH350 or a TH400, you'll need to use JET's 4L80E TransConversion kit, which allows the four-speed to function with a carbureted, fuel-injected, or diesel engine. The kit includes a stand-alone computer with special software that will provide proper shift points, shift quality, and a torque converter lock-up function. Additionally, the kit's wiring harness has a built-in diagnostic port that allows the use of scanning tools for transmission monitoring.
Ford Automatic Transmission Identification Guide
As with the automatic transmissions from GM and Dodge, the various models of Ford transmissions can be easily identified by checking the shape of the pan gasket and the number of retaining bolts. Ford's three-speed transmissions are the C4 and the C6; the four-speed overdrive Ford transmissions are the E4OD, the AOD, and the AODE. Small-block Blue Oval engines installed in light trucks from 1967 to 1981 were backed by the C4 tranny. Trucks built during 1967 to 1969 were equipped with the C4-67 version; trucks built between 1970 to 1981 used the C4-70 trans. Both C4 transmissions are similar in outward appearance and both C4s have a vacuum modulator located on the passenger side of its case toward the rear section. FoMoCo's big-block engines (except diesel models) were equipped with the C6 transmission from 1967 to 1996. The C6 is identified by its large overall size (compared to a C4) by the 18 fasteners holding the pan and by the vacuum modulator mounted on the case's passenger side toward the rear. The four-speed overdrive AOD, used on many '83-to-'93 trucks, is equipped with a TV rod on the transmission's passenger side near the rear. Ford's E4OD trans was installed behind various small-block and big-block engines during 1989 to 1998, with an upgraded version introduced in 1998. The E4OD case has an electrical connector and a wiring harness located on the passenger side near the rear section. The '91-to-2000 Ford trucks may have also been equipped with the AODE, the electronically controlled version of the AOD. The AODE also uses an electrical connector and a wiring harness on the rear passenger side area.
Ford Automatic Transmission Gear Ratios
JET's Aod/Aode four-speed overdrive transmission is an excellent choice for Ford truck owners seeking a modern transmission for their haulers that can withstand the rigors of towing, a high-horsepower engine, and off-road use. The JET modifications include a nine-pack clutch set in the three-to-four clutch drum; a double-seal accumulator; heavy-duty bands and clutches; a performance-modified valvebody; and performance servo pistons. The JET AODE overdrive is available with a complete installation kit and can reliably handle as much as 400 horsepower.
If you need a four-speed overdrive transmission for a high-torque Ford engine, try JET's E4OD. The E4OD features a steel gear train in place of the OE aluminum gears; clutch drums that are machined to accept additional heavy-duty clutches; a performance solenoid pack and valvebody; and stronger clutch bands. JET's E4OD is suited for either gas or diesel engines.
Beyond getting a FoMoCo E4OD four-speed overdrive trans physically mounted in place of your truck's three-speed, you have to ensure that the transmission's electronic functions operate as required. To get the E4OD under control, install JET's TransConversion kit, a stand-alone computer system with an integral wiring harness that is calibrated to provide torque converter lock-up function, accurate shift points, responsive shift quality, and is completely compatible with your Ford's existing emission controls. The kit allows the E4OD to function behind a carbureted, fuel-injected, or diesel engine; its wiring harness features a diagnostic port to allow the use of a scanning tool to monitor transmission condition.
Dodge Automatic Transmission Identification Guide
When compared to GM and Ford, Dodge/Chrysler's use of the Torqueflite/ Loadflite A-727 and the A-904 three-speed automatic transmissions makes identification and swapping a snap. Fullsize Dodge trucks - powered by either a small-block or a big-block engine - were equipped with the A-727 tranny from '66-to-'93. The A-727 sports a vacuum modulator near the front of the case on the passenger side.The 518/618 Mopar four-speed overdrive transmission uses a wiring harness and an electrical connector on the driver side near the rear of the case.
Dodge Automatic Transmission Gear Ratios
To make the switch from three-speeds to four-speeds, JET Performance Products offers a performance-built 518/618 transmission to make the install a straightforward bolt-in affair. The 518 overdrive trans is for V-8 engines; the 618 is set up for V-10 and diesel powerplants. Both transmissions are thoroughly inspected before being completely built using performance components, including modified oiling circuits, heavy-duty bands and clutches, and a modified valvebody. These mods allow the 518/618 trans to handle increased engine torque and deliver improved shifting performance and acceleration. The robust design of the JET 518/618 is especially suited to towing applications because of its extreme-duty modifications.
In order to get the 518/618 four-speed functioning appropriately when installed in a Dodge truck previously equipped with a three-speed trans, you'll need JET's TransConversion kit, which adds a vacuum control system that signals an electronic switch, which activates the 518/618's Fourth gear torque converter lockup, which is normally controlled by the engine's computer on late-model Dodge trucks. The TransConversion kit also provides driver-adjustable lock-up settings for the converter.
The Overdrive Alternative: Gear Vendors' Add-On Gearbox
As with many mechanical upgrades and modifications, there exists an alternative method to give your truck an Overdrive Fourth gear: an add-on auxiliary gearbox. The name synonymous with performance auxiliary transmissions is Gear Vendors, and the company has an incredibly strong, bolt-on overdrive/underdrive unit that will turn a three-speed automatic transmission into a true six-speed trans, although the focus of this story is the Gear Vendors' Overdrive capabilities.
The Gear Vendors' Under/Overdrive auxiliary transmission is complex in design and function, but it's a well-engineered unit that's capable of handling as much as 1,200 hp in regular trim and even more with special factory modifications. It's beyond the scope of this story to fully illustrate the complete inner workings of the Gear Vendors' unit, so let's concentrate on the changes in gear ratio the Under/Overdrive transmission effects.
Used strictly as an overdrive on a three-speed automatic transmission, the GV unit will provide an overdrive ratio of 0.78, which is on par with the overdrive ratios provided by Dodge, Ford and GM OE four-speed overdrives. In Overdrive, engine speed is reduced by approximately 22 percent, which makes super-stiff final drive gears liveable on a day-to-day basis. Engaging the Overdrive is akin to changing a set of axle gears with a 4.56 ratio into gears with a 3.55 ratio. That 22 percent gear reduction (Overdrive) makes for more efficient highway fuel mileage, and allows you to gear your off-roader ultra low for torque and to turn large-diameter tires while maintaining the ability to cruise without over revving the engine.
In operation, the Gear Vendors' transmission can be set into either an automatic or a manual mode. In automatic mode, the GV unit automatically shifts into Overdrive after the engine is run through Third gear. The GV unit is set to shift into Overdrive at approximately 47 mph, but the Overdrive shift speed can be tailored at the factory to your application.Basically, the GV unit makes a three-speed trans function, similar to a four-speed, with no driver interaction required. When the transmission downshifts, such as when the vehicle comes to a stop, the GV Overdrive is automatically turned off, returning the transmission to its non-Overdrive status.
For a more advanced use of the Gear Vendors' Under/Overdrive unit, a driver can select the manual or the interactive mode. In the manual mode, a floor-mounted foot switch or a thumb trigger on the shift lever controls the unit's functions. Starting in First gear with the GV unit in underdrive, Overdrive is engaged as the vehicle reaches the top of First gear, thus splitting the gap between First and Second with an additional gear. This gear splitting can be used between all of the forward gears, thus enabling a three-speed transmission to function as a six-speed.
Gear Vendors makes a wide selection of auxiliary gearboxes for all manner of 2WD and 4x4 applications. Some GV units attach to the rear of the transmission's bellhousing; other versions use a divorced gearbox coupled with short driveshafts. If you're after something truly trick, check out Gear Vendors' Under/Overdrive units for trucks equipped with a factory Overdrive four-speed, which delivers unbelievable gear splits and a double Overdrive.
For More Editorial And Product Information, Plus