How To Improve MPG & Performance With An Overdrive Transmission - CHP How It Works

Going Broke Filling Up Your Hot Rod? Here’s How to Improve MPG and Performance With an Overdrive Trans

Stephen Kim Jun 15, 2011 0 Comment(s)
View Full Gallery

Six-Speed Options

Stan Poff: The 6L80E came from the factory with GM’s torque management programming built into the vehicle’s electronics, which manipulates engine tuning to reduce

torque output in an effort to increase transmission longevity. That alone should give you a good idea as to the problems it faces. I personally know GM Proving Ground engineers who have tried to step up the performance of vehicles equipped with the 6L80E, and they haven’t been very successful. TCI has been analyzing and planning some upgrades for the 6L80E in order to make it live behind added horsepower. To the contrary, the 4L80E was a very good transmission from the factory. Like most all of the transmissions coming from GM, there are always upgrades made during their manufacturing runs that make each transmission a little better, hence the 4L85E has been improved over the 4L80E with stronger planetaries and improved oiling circuitry. Some refer to the 4L80E as the TH400 with an overdrive ratio, as much of the internals are interchangeable. TCI has a complete line of 4L80E transmissions in the Street Fighter and Super Street Fighter trim levels. These are better constructed with many aftermarket components that will allow it to handle much more horsepower and increase durability for even normal daily driven vehicles.

For a best-of-both-worlds solution, TCI developed the 6x Six Speed transmission, which was born out of the 4L80E. To accomplish this, TCI manufactured proprietary components, made fluid circuitry changes, and engineered two more gears into the transmission. The TCI 6x Six Speed transmission features ratios of 2.97:1, 2.23:1, 1.57:1, 1.18:1, 1.00:1, and 0.75:1. In addition to shifting through all six gears on its own with the gear selector in the “OD”, the 6x Six Speed can also be shifted manually with paddle shifter or a two-button shifter with hard wires by installing a toggle switch that allows the controller to go back and forth from automatic mode to manual mode. This trans is a real workhorse, and current models consistently endure 850-1,000 hp with ease. Furthermore, TCI has developed better components and upgrades expected to take it to the 1,500hp range shortly. It also uses modular bellhousings, so it can be easily adapted to many application offered by GM, Ford, and Chrysler.

Factory Overdrive Alternative

Rick Johnson: A Gear Vendors overdrive unit is a supplemental planetary assembly that bolts to the output shaft of the transmission in place of the tailshaft housing. It features a 0.78:1 ratio for a 22 percent reduction in cruise rpm, essentially converting a two- or three-speed trans to an overdrive trans. So if your car turns 3,500 rpm on the freeway, it will lower rpm by nearly 800. Gear Vendors overdrive units are available for Powerglide, TH350, and TH400 transmissions. In most GM applications, installing a Gear Vendors unit requires little to no modification to the trans tunnel at all. The factory crossmember can be retained, and the only other change necessary is shortening the driveshaft by 121/2 inches. The big advantage of running a Gear Vendors unit on an earlier two- or three-speed GM trans is that the older transmissions are stronger, more reliable, easier to repair, and have less parasitic loss than factory overdrives. They also eliminate the need for electronics and lockup converters. Not only are Gear Vendors units great for improving fuel mileage, but they can handle 2,000-plus horsepower in racing applications. Once a Gear Vendors unit has been installed, it is engaged with a shift button that’s usually mounted on the shifter. Once you click the button for overdrive, a green light on the dash lets you know it has been engaged. Simple electronics that come with the kit shut off the overdrive when you come up to a stop or reach low speeds. The electronics also give you an AutoDrive and AutoLaunch features, which are cool for cruising or racing.” CHP


Gearstar Performance Transmissions
Akron, 44308
TCI Automotive
Ashland, MS 38603
Gear Vendors
El Cajon, CA 92020
Monster Transmission
Brooksville, FL 34601