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Overdrive and Underdrive on a 1986 Chevy Dually - Gear-Grabbin' Goliath
Gear Vendors' Overdrive/Underdrive Unit Improves Mileage And Top Speed For Heavy-Duty Haulers
From the April, 2004 issue of Sport Truck
By Mike Finnegan
Photography by Mike Finnegan
Since you're a Sport Truck reader, we'll assume that the way your truck looks is as important as the way it performs. If you only cared about how your truck towed a trailer, or hauled hay bales, then you probably wouldn't even pick up Sport Truck. But we know you care about the way your truck looks and runs whenever you're cruisin' it, towing with it, or generally mobbin' around town in it. Since the lowering craze has reached epidemic proportions, where even a 1-ton dualie is not safe from being hammered flat on the pavement, we thought we should help you alleviate one of the problems associated with laying one of these monsters out. Most any dualie that lays completely flat these days is running tires that are several inches shorter in overall diameter than the ones that came from the factory. Take into account the fact that a majority of these trucks came with 4.11 gearing and you have a truck that screams down the highway at more than 3,500 rpm at just 55 mph. Fuel economy and engine life go right out the window under these conditions. Sure, your truck runs like a chicken with its head cut off, but when you merge on the freeway, you're left in the dust by any vehicle that can break the speed limit. Gear Vendors can fix this problem - as well as a few others. The company's Overdrive/Underdrive unit is a simple remedy that will not only enable your truck to cruise comfortably, but will also tow larger payloads and gain better fuel economy depending on how you drive it. In just a day, you can be up and running, splitting gears and burnin' rubber, or just motoring down the freeway at your desired pace. To find out more, read on.  1. Here is our guinea pig,...  1. Here is our guinea pig, an '86 Chevy Crew Cab dualie that is airbagged, body-dropped, and running low-profile 16-inch tires. With a rearend stuffed with 4.11 gears, this truck became unfriendly on the freeway once the custom work was performed. The Gear Vendors unit will fix this by giving us a much-needed overdrive gear and reduce our cruising rpm by 700. After we made the journey to Gear Vendors in Chula Vista, California, the dualie was driven up onto a lift. Gear Vendors installs its products in recreational vehicles, motorhomes, light pickups, and heavy-duty models. Almost any truck on the road can be improved by Gear Vendors.  2. Before removing the driveshaft,...  2. Before removing the driveshaft, an angle finder was used to check the angle of it. This was done so that when the install was finished, we could verify that everything stayed the same.  3. Marks were made at both...  3. Marks were made at both ends of the driveshaft so that after it was shortened it could be reinstalled just the way it was removed.  4. The rear section of the...  4. The rear section of the two-piece driveshaft was separated from the rear axlehousing by unbolting the U-joint.  5. The carrier bearing supports...  5. The carrier bearing supports and connects the two-piece driveshaft. In this photo, the rear driveshaft is unthreaded from the carrier bearing and the driveshaft is removed.  6. The carrier bearing is...  6. The carrier bearing is then unbolted from the crossmember with an impact gun and 5/8-inch wrench.  7. With the carrier bearing...  7. With the carrier bearing removed, the front of the driveshaft is then unbolted from the back of the transmission.  8. Finally, the front half...  8. Finally, the front half of the driveshaft is removed from the truck. The two driveshaft pieces are then taken to a driveline shop to be shortened and balanced. This is done because the Gear Vendors unit replaces the tailshaft of the transmission with an adapter and is significantly longer than the stock transmission.  9. While the driveshaft is...  9. While the driveshaft is being shortened, the electronic portion of the Gear Vendors unit is set up for installation. The system consists of a computer, on/off switch, signal generator, and manual override switch.  10. The manual/automatic switch...  10. The manual/automatic switch is mounted to the bottom of the dash, in an accessible location. This switch toggles between gear-splitting mode and automatic overdrive mode. Gear splitting mode allows the driver to select between First gear, First Overdrive, Second gear, Second Overdrive, Third gear, and Third Overdrive, effectively giving the driver a six-speed transmission. The Automatic Overdrive mode will automatically engage the Gear Vendors unit at 47 mph and put it in overdrive.  11. The computer is mounted...  11. The computer is mounted beneath the dash of the truck and will keep count of the number of times the unit shifts into Overdrive.  12. The on/off switch is mounted...  12. The on/off switch is mounted on the floor, to the left of the brake pedal. It functions just like an old-school high/low-beam headlight switch. When in Manual mode, you just tap the switch to toggle between Overdrive and Underdrive gear settings.  13. These cables attach to...  13. These cables attach to the computer and are routed through the firewall. Eventually they will attach to the signal generator, underneath the truck.  14. At this point, the wiring...  14. At this point, the wiring is nearly complete and it's time to install the Gear Vendors unit.  15. A transmission jack is...  15. A transmission jack is placed beneath the tailshaft of the transmission for support.  16. The yoke is the first...  16. The yoke is the first item to be removed from the back of the transmission. It will be replaced when the driveshaft is reinstalled.  17. Next, the tailshaft housing...  17. Next, the tailshaft housing is removed from the back of the transmission. This is accomplished with an impact gun and 1/2-inch socket. The stock housing will be replaced with the Gear Vendors adapter.  18. This is what the back...  18. This is what the back of the transmission looks like once the tailshaft housing is removed.  19. On the left is the Overdrive/Underdrive...  19. On the left is the Overdrive/Underdrive unit, and on the right is the transmission adapter. Gear Vendors manufactures adapters for nearly any truck's transmission.  20. Before the unit can be...  20. Before the unit can be installed onto the transmission, the adapter plate must first be installed. The adapter is bolted to the rear of the transmission using the original mounting bolts.  21. A mounting gasket is placed...  21. A mounting gasket is placed onto the adapter, then a coupling is inserted onto the tailshaft. A straight edge is placed across the mounting surface of the adapter to see if there is a gap between it and the coupling.  22. If there is a gap, the...  22. If there is a gap, the spacers included in the kit must be installed to ensure the coupling is installed properly. By sliding the spacers one at a time between the coupling and straight edge, we can determine how many are necessary.  23. Once we've determined...  23. Once we've determined the number of spacers needed, the coupling is removed from the tailshaft and the spacers are placed inside of it. The coupling is then reinstalled onto the tailshaft.  24. Before the Underdrive/Overdrive...  24. Before the Underdrive/Overdrive unit is installed onto the adapter, it is quickly inspected and the 'shaft is turned to make sure everything moves freely.  25. The unit is then positioned...  25. The unit is then positioned onto the adapter housing. It should fit snugly over the mounting studs.  26. Lock nuts with nylon inserts...  26. Lock nuts with nylon inserts secure the unit to the adapter. They are tightened all the way around the unit using a 9/16-inch wrench.  27. The speedometer cable...  27. The speedometer cable that was attached to the transmission is now removed and this signal generator is installed. The wires you see dangling from the signal generator are attached to one of the cables we ran through the firewall in photo 15.  28. On the other end of the...  28. On the other end of the signal generator, a new speedo' cable is installed and threaded into the Gear Vendors unit.  29. The Gear Vendors unit...  29. The Gear Vendors unit shifts after receiving input from the computer. The input is carried through the second cable that we ran in photo 15. That cable gets hooked up to this solenoid, located on the left side of the unit.  30. The wiring and install...  30. The wiring and install are nearly complete. After installing our newly shortened and balanced driveshaft, the Gear Vendors unit is filled with transmission fluid and given a quick check out. Once the unit was given a thumbs up by the technician, we were ready for the open road. Whenever we towed a heavy load and encountered a steep grade, we were able to split gears and find our ideal towing speed. On the open highway with no load, the new Overdrive gear worked superbly at bringing out beast to a safe cruising rpm, and we found a noticeable difference in fuel economy.
Gear Vendors Inc.
1717 N. Magnolia Ave.
El Cajon
CA
92020
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