Installing a Gear Vendors Overdrive In Your Muscle Car

The article featured on this page is from the June 2011 issue of Auto Enthusiast Magazine.

Installing a Gear Vendors Overdrive in a First-Gen F-Body

Story Larry Weiner

We live in a day and age of rapidly escalating fuel costs, to say nothing of the constant outcry from environmentalists about the carbon footprint, global warming and air pollution caused by vehicles powered by internal combustion engines.

It’s no secret that the vintage and muscle cars are often held up as examples of what is damaging the environment. The ever-tightening emissions standards and mandatory smog testing of older vehicles clearly illustrates the direction where this is headed.

Most of the data used by government bureaucrats and environmentalists to blame our collector cars for harming the environment is flawed for a wide variety of reasons. Unlike regular daily drivers, the annual mileage of most classics is nominal, often no more than 500 to 1,000 miles per year. In addition, these vehicles receive a high degree of maintenance and are generally driven very carefully. Regardless of the efforts of influential organizations such as SEMA and Goodguys, it’s safe to say that the pressure on the collector car hobby is not going to ease up.

With that thought in mind, there is something we can do that will dramatically improve the performance of our vehicles while simultaneously further discrediting many of the arguments against them. A proven driveline component is readily available to significantly enhance fuel economy and reduce greenhouse gas emissions by 25 percent while increasing vehicle performance. It’s called an overdrive.

We take it for granted today in new vehicles, as most manufactured since the early 1980s have been equipped with transmissions that include an overdrive. The majority of classic cars and virtually none of the mighty muscle cars from the golden era were equipped with an overdrive. The result is that these vehicles, especially those equipped with high numerical final drive ratios, deliver marginal fuel economy because they operate at elevated engine rpm, especially when driven at highway speeds. Installing an overdrive is the practical solution to this problem and the good news is that it’s easy to do.

This story will illustrate how simple it is to make your vintage Chevy a modern highway cruiser thanks to a Gear Vendors overdrive. This is the most popular overdrive conversion for factory transmissions, with over 100 applications. The Gear Vendors unit is manufactured right here in the United States, just like your classic Chevy.

Our 1967 Camaro SS350 is equipped with its original Muncie M20 four-speed manual transmission and 12-bolt differential with a 3.73 gear set. Since fourth gear in the transmission is a 1:1 ratio, with the 3.73 gear, every time we reached 55 miles per hour, the Camaro felt like it was ready to be shifted to the next higher gear. The problem was that we were already in fourth, and there weren’t any more gears left.

Although the vehicle only weighs 2,940 pounds, which is light by today’s standards, the high gearing resulted in a busy engine and mediocre fuel economy. Here in Southern California, the 18 wheelers run faster than the double nickel in the slow lane. Because we like life in the fast lane, and certainly wanted better fuel economy, to say nothing of doing our part to help save the environment, we decided to install a Gear Vendors overdrive in the Camaro. Gear Vendors offers a 22 percent overdrive, turning our 3.73 gear into a 2.91 final drive. This lowered the rpm at 70 mph from 3,700 to 2,886, making our Camaro SS far more freeway friendly. Goodbye semi trailer trucks, hello fast lane.

Living life in the fast lane will not be limited to the freeway. While the Muncie M20 is a robust design and was a state-of-the-art transmission in the 1960s, today a four-speed is something of an anachronism. The Gear Vendors overdrive brings this legendary muscle car transmission into the 21st century. It not only provides the Camaro with an overdrive, but it also enables you to split the gears in the Muncie, and take full advantage of the benefits of both the transmission and the overdrive.

If you really want to maximize the versatility of the combination, get ready to shock your friends and competitors by starting in first gear like a conventional manual. Upshift using the overdrive to first over, a lightning shift that will make your vehicle act and sound just like an automatic. Then, leaving it in overdrive, shift normally to second and third. During the shift to fourth, press the button on the Gear Vendor shift knob and you’ll get the regular fourth gear ratio and then follow that up with fourth over by pressing the button again. The result is a fully functional six-speed transmission that retains the tactile feel and precision you expect from a Muncie, complemented by the advantages of the Gear Vendors overdrive.

Follow along with us as the master technicians at Gear Vendors install one of their under/overdrive units on the 1967 Camaro and turn it into a muscle car with a newfound ability to compress time and space while achieving fuel economy that will rival that of many small import cars.


1 These parts, every part for installation, are included in Gear Vendors kit.

2 The original Muncie M20 transmission with a vintage 1960s Hurst Competition Plus Shifter instead of the Muncie shifter that would have been installed on the Camaro at the factory. This Hurst shifter has a unique mounting bracket not used for many years and is a good indicator that it has been on the Camaro since it was nearly new.

3 The stock driveshaft is measured by a Gear Vendors tech prior to removal.

4 Remove the nuts and lock washers that secure the two straps on the universal joint at the differential, then remove the driveshaft.

5 Unscrew the speedometer cable retaining nut from the transmission and remove cable.

6 Remove the two bolts from shifter where shift arm is attached. Next, remove factory reverse switch from bracket on side of transmission

7 Position the transmission jack under the transmission. Remove the four bolts that secure the transmission brace to the front subframe. Then, remove the two bolts that secure the rear transmission mount to the transmission brace.

8 Raise the transmission slightly by using transmission jack to take the weight off of the transmission brace. Then remove the transmission brace by sliding it at an angle and pull it free of subframe.

9 Remove four bolts at the front of transmission where it is mounted to the bellhousing.

10 Place a jackstand under the oil pan to support the engine. Use a block of wood to distribute the weight and prevent damage to the oil pan.

11 Carefully pull the transmission straight back from the bellhousing to avoid damaging the clutch.

12 The Muncie M20 is taken to the bench ready for removal of stock tailhousing.

13 The shifter must be removed.

14 The next step is removal of roll pin (drive out with punch) that retains reverse shift shaft. With the roll pin removed, grasp reverse shift shaft with a wrench and pull outward.

15 Remove four bolts that attach the tailhousing to transmission main case.

16 Gently tap the tailshaft with a soft-faced mallet to separate tailhousing from transmission main case.

17 With the tailhousing removed, clean the surface in preparation of installing new gasket.

18 Here’s a comparison of the original Muncie tailhousing with Gear Vendors one-piece and two-piece adapters. The one-piece housing is the taller of the two GV adapters. The two-piece adapter is used in 1963 thru 1981 Corvettes to accommodate the transmission crossmember that cannot be removed, while the one-piece is used in GM F-Bodies. Note that while the one- and two-piece GV adapters are longer than the original Muncie tailhousing, the tunnel of a 1967 – 1969 GM F-Body accommodates the length of the longer adapter and Gear Vendors unit with no clearancing required.

19 The reverse arm in the Muncie tailhousing must be removed and reused. Here’s a view of the reverse arm, detent spring and ball removed from Muncie tailhousing.

20 On the Gear Vendors adapter, there’s a hole for the detent spring and ball. Install it, then slide the reverse arm over the detent spring and ball. This is the reverse arm fully installed.

21 Install the Gear Vendors adapter, then reinstall the roll pin retainer for reverse gear arm.

22 Reinstall the original bolts in Gear Vendors adapter and torque to 20 lbs-ft.

23 Install rubber O-ring seal prior to bolting on second section of Gear Vendors adapter, then put on the second section. Use a small amount of Loctite on bolts used to attach the second section.

24 The Gear Vendors single piece rear adapter is installed on Muncie M20 transmission for F-Body application.

25 Gear Vendors’ splined coupling is used to connect the transmission output shaft with the Gear Vendors Overdrive unit.

26 Install the coupling and check tolerance to face of adapter with gasket installed.

27 Using a straight edge, determine the number of shims necessary to equal the distance between the top of the coupling and the face of the gasket on the GV adapter. In this case, four shims were used. We placed the shims inside the coupling recess.

28 Install the coupling on the transmission output shaft.

29 Check clearance to confirm that the coupling and face of the gasket on the adapter match up.

30 After the shifter is installed on the transmission and the shift arms are adjusted, install the transmission in vehicle, and replace the transmission mount and crossmember.

31 Note the ample space around the Gear Vendors transmission adapter when it’s installed in the vehicle.

32 Install the gasket on the Gear Vendors overdrive unit. Temporarily install Gear Vendors overdrive unit onto the transmission adapter so that measurements can be taken for a new driveshaft.

33 Measure for the new driveshaft. Be sure to allow 5/8 of an inch clearance from base of the driveshaft yoke to back of Gear Vendors overdrive unit to prevent bottoming out.

34 Remove the Gear Vendors Overdrive from the vehicle and install the speedometer drive and gear. Use white lithium grease on rubber seal. After drive is installed, secure with retaining bolt. Note use of 90-degree speedometer drive to clear the transmission tunnel.

35 Install Gear Vendor overdrive unit on transmission adapter and tighten all nuts.

36 Note the difference in length between the original driveshaft and the new unit.

37 Install the Gear Vendors computer inside vehicle. In the case of the Camaro, the computer was mounted just beneath the heater core, under the carpeting. Connect wiring to computer per instructions.

38 Gear Vendors signal lights are mounted on a small plate at bottom of dash using an existing hole and screw. The green light indicates unit is engaged and red light indicates unit is on.

39 The speedometer cable extension is connected to speedo drive on the Gear Vendors overdrive unit. Note the wires connected to GV solenoid.

40 Connecting original speedometer cable to GV extension cable.

41 The new driveshaft is installed in the vehicle. Note the 5/8-inch travel for the yoke on the new driveshaft as per GV instructions.

42 Fill Gear Vendors Overdrive unit with 28 ounces of oil. Note the ample space around the Gear Vendor unit when installed. No modifications to the floor or tunnel of any kind were required for the installation of the unit in the Camaro.

43 A view of the completed installation.

44 It is almost impossible to detect the installation in the interior. The shift knob with the button and the small plate under the dashboard with two indicator lights are the only clues that the Camaro is now equipped with overdrive. Note that the shifter is still in its original factory location and that it fits perfectly in the console.

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