The Gear Vendors under/overdrive system has long been a popular enhancement for enthusiasts seeking to improve both performance and efficiency in their vehicles. By effectively converting a three- or four-speed transmission into a six-speed unit, this system allows for greater flexibility in gear selection, improved fuel economy, and enhanced driving dynamics. This article provides a detailed breakdown of the installation process, key considerations, and performance benefits based on information from official installation guides, user reports, and technical documentation provided by Gear Vendors and third-party installers.
Gear Vendors’ under/overdrive is designed to work with a wide range of transmissions, including the TH400, TH350, and other similar units. It operates by splitting the existing gear ratios, creating six forward gears—First, First-overdrive, Second, Second-overdrive, Third, and Third-overdrive—without the need for a clutch. This allows the engine to remain within its power band during acceleration and cruising, reducing wear and improving efficiency.
The system offers two primary modes of operation: automatic and manual. In automatic mode, the overdrive can be programmed to engage at a set speed, typically between 45 and 65 mph, depending on the user's preference and vehicle configuration. In manual mode, the driver can split gears by shifting into overdrive at any time, providing more control over the vehicle's performance. Additionally, the unit can be programmed to launch at a specific speed in overdrive, which is particularly useful in drag racing applications.
Installation of the Gear Vendors under/overdrive is described as relatively straightforward in most applications, though some vehicles may require additional modifications to accommodate the unit's dimensions. The process generally involves removing the factory tailshaft housing, attaching the Gear Vendors adapter and overdrive unit, and shortening the driveshaft to the appropriate length. The system also requires the installation of wiring for the electronic control unit (ECU), a foot-activated or gear-shift-mounted switch, and a dash-mounted control panel. In some cases, minor modifications to the vehicle’s tunnel or crossmember may be necessary to ensure proper clearance.
This article will walk through the installation steps in detail, discuss key considerations such as compatibility and clearance, and highlight the performance and economic benefits that users can expect from installing a Gear Vendors under/overdrive unit.
Installation Overview
The installation of a Gear Vendors under/overdrive unit involves both mechanical and electrical components. According to the manufacturer’s documentation and user reports, the process can be broken down into several key steps.
Mechanical Installation
Remove the Driveshaft and Transmission Housing
The first step involves removing the original driveshaft and the factory transmission tailshaft housing. This allows access to the back of the transmission for attaching the Gear Vendors unit. In most cases, the original driveshaft will need to be shortened to accommodate the new overdrive unit. Users are advised to take the driveshaft to a professional shop for this modification, as it requires precise cutting and welding to ensure proper balance and alignment.Install the Gear Vendors Adapter and Overdrive Unit
The Gear Vendors unit is mounted to the transmission using a replacement housing that bolts directly to the transmission case. The unit is secured with eight nuts, and it is important to ensure that all bolts are tightened to the manufacturer’s specifications. The unit is designed to be compact and fits closely with the original transmission, though some vehicles may require minor adjustments to the tunnel or crossmember for clearance.Mounting and Clearance Adjustments
For vehicles with tight tunnel areas—such as certain street rods or custom builds—users may need to make minor modifications to ensure proper clearance. This can include shortening the upper rear case studs or creating a small dimple in the tunnel to allow the overdrive unit to fit. Some users have reported that installing a G-Force crossmember designed specifically for the Gear Vendors unit can help avoid tunnel modifications altogether.Reinstall the Crossmember and Trans Mount
Once the overdrive unit is securely mounted, the transmission crossmember and trans mount can be reinstalled. This step is important for ensuring that the transmission is properly supported and aligned with the rest of the drivetrain.Install the Driveshaft
After the overdrive unit is in place, the modified driveshaft can be reinstalled. It is essential to ensure that the driveshaft is properly balanced and that the yoke is correctly aligned with the overdrive unit to prevent vibration and wear.
Electrical Installation
Connect the Speedometer Cable and Electronics
The Gear Vendors system requires a speedometer cable adapter to maintain accurate speedometer readings. The cable is connected to the overdrive unit and routed to the speedometer. Additionally, the unit requires a power source, which is typically connected to the vehicle’s electrical system via the battery or an existing fuse box.Install the Foot-Activated or Gear Shift Switch
The system includes a foot-activated or gear shift-mounted switch that allows the driver to engage or disengage the overdrive manually. This switch is connected to the control unit and must be wired correctly to function as intended.Mount the Control Panel and Connect the ECU
A dash-mounted control panel is installed to provide the driver with options for switching between automatic and manual modes. The control panel is connected to the ECU, which manages the shifting logic and programming settings. The ECU is typically installed in a convenient location under the dashboard or in the engine compartment.Programming and Testing
Once all components are installed, the system must be programmed to match the vehicle’s specifications. This includes setting the overdrive engagement speed, configuring the shift pattern, and ensuring that the unit functions correctly in both automatic and manual modes. A test drive is recommended to verify that the system is operating as expected and that there are no issues with shifting or speedometer readings.
Key Considerations and Compatibility
Before installing a Gear Vendors under/overdrive unit, it is important to consider several factors to ensure compatibility and proper function.
Vehicle Compatibility
Gear Vendors overdrive units are designed to work with a wide range of transmissions, including the TH400, TH350, and other similar three- or four-speed units. However, it is important to verify that the vehicle’s transmission is compatible with the unit before beginning installation. The manufacturer provides detailed compatibility charts and technical support to help users determine whether their vehicle is suitable for the system.
Clearance and Tunnel Modifications
As mentioned earlier, some vehicles may require minor modifications to the tunnel or crossmember to accommodate the overdrive unit. This is particularly true for street rods or custom builds with tight tunnel areas. Users are advised to check for clearance before installation and to make any necessary adjustments to prevent interference with the unit.
Driveshaft Modifications
Shortening the driveshaft is a critical step in the installation process. Users are encouraged to take the driveshaft to a professional shop for this modification, as improper shortening can lead to balance issues and vibration. The manufacturer provides instructions for shortening the driveshaft and recommends using a new yoke to ensure proper alignment.
Electrical Integration
The electrical components of the system must be integrated carefully to ensure that the overdrive functions correctly. This includes connecting the speedometer cable, wiring the control unit, and installing the foot-activated or gear shift switch. Users should ensure that all electrical connections are secure and properly insulated to prevent short circuits or interference.
Warranty and Support
Gear Vendors provides a 2-year unlimited mileage warranty on all its products, except for diesel manual truck and commercial applications. The company also offers a money-back guarantee, allowing users to return the product if it does not meet their expectations. Additionally, the manufacturer provides free technical support, diagrams, and installation guides to assist users with the installation process.
Performance and Economic Benefits
One of the primary reasons users choose to install a Gear Vendors under/overdrive unit is the performance and economic benefits it offers.
Improved Fuel Economy
By allowing the engine to operate at lower RPMs during highway driving, the overdrive system can significantly improve fuel economy. This is especially beneficial for vehicles with high-torque engines that tend to rev higher at cruising speeds. Users have reported measurable improvements in fuel efficiency after installation, particularly when driving at higher speeds for extended periods.
Enhanced Driving Experience
The ability to shift into overdrive at any time, whether in automatic or manual mode, provides greater control over the vehicle’s performance. This is particularly useful in situations where maintaining engine speed is important, such as when driving on hilly roads or during towing applications. The unit’s ability to split gears also allows for smoother transitions between gears, reducing stress on the drivetrain and improving overall driving comfort.
Performance Enhancements
In performance applications, the Gear Vendors system offers the ability to launch in overdrive at a set speed, which can improve 0-to-60 times and reduce tire spin. This is particularly useful in drag racing applications, where maintaining engine speed and traction is critical. Additionally, the system’s ability to keep the engine in its power band during acceleration allows for greater performance without the need for higher gear ratios or engine modifications.
Durability and Reliability
Gear Vendors overdrive units are built to withstand the rigors of both street and track use. The company uses high-quality materials and precision manufacturing techniques to ensure that its units are durable and reliable. Users have reported that the units operate smoothly and require minimal maintenance, making them a long-term investment for enthusiasts and professionals alike.
Conclusion
The Gear Vendors under/overdrive system is a versatile and effective solution for enhancing the performance and efficiency of vehicles equipped with three- or four-speed transmissions. By converting a standard transmission into a six-speed unit, the system offers greater flexibility in gear selection, improved fuel economy, and enhanced driving dynamics. The installation process, while requiring some mechanical and electrical work, is described as relatively straightforward in most applications.
Key considerations such as vehicle compatibility, clearance requirements, and driveshaft modifications must be addressed before installation to ensure proper function. Additionally, the system’s performance and economic benefits make it a valuable addition for both daily drivers and performance enthusiasts.
With the support of detailed installation guides, technical documentation, and customer service from the manufacturer, users can confidently install and operate the Gear Vendors under/overdrive system to maximize the potential of their vehicles.