Mustang Illustrated jun01

Words: Bob McClurg
Images: by the author
Reprint from Mustang Illustrated Magazine, June 2001

Usually, when someone starts talking about overdrive or underdrive transmissions, you draw a mental picture of a huge motorhome or some piece of heavy-duty road equipment lumbering down the highway. The fact of the matter is that overdrive transmissions have been used in passenger cars for close to 60 years. Cadillac had them. Ford had them. Chrysler had them. Even AMC had them.
Then, in the early 1960s, when high-performance became a prime selling point, overdrive transmissions faded from view, if only temporarily.
Today, the factory overdrive transmission has made a huge comeback, with models like the GM 700-R4 and the Ford AOD and AOD-E. But why?
Fuel economy, plain and simple. The four-speed transmission has enjoyed a resurgence in popularity primarily because its additional cog and lock-up-torque-converter design help the cars that employ it meet the EPA’s Corporate Average Fuel Economy (CAFE) guidelines.
However, unlike their predecessors, these factory overdrives are electronically activated, relying on computer-controlled shift points. Unfortunately, the torque converters in these new transmissions seem to lock up at the most inappropriate times, sacrificing performance in the interest of fuel efficiency. Such a deal, huh?
When it comes to the street-machine set, both GM’s and Ford’s latest-electronic-overdrive gearboxes have experienced phenomenal popularity with transmission swappers. However, while these units can provide enthusiasts with the affordable fuel economy they desire, there is actually very little, if anything, to be gained in the performance department.

This is the Gear Vendors Planetary Under/Overdrive installation kit (P/N 3D0040) for 1964 1/2 to 1973 Mustangs equipped with the Ford C-4 three-speed automatic transmission. The kit includes the transmission, transmission adapter and coupler assembly, electronic control module and foot control switch, speedometer-cable-adapter assembly, all the necessary wiring and both long and short versions of the assembly book. Not included in the kit is a new driveshaft, which you’ll have to have made.

More grim reality: These electronically managed overdrives were not meant to handle 400-plus-horsepower loads without internal modifications, which can drive the cost of an overdrive transmission conversion even higher.
The solution is to install an aftermarket auxiliary overdrive transmission, like Gear Vendors, Inc.’s popular Under/Overdrive planetary unit. This trans bolts directly to the back of your existing automatic-transmission case (or to a manual box, using the company’s adapter), and provides you with twice the gear ratios of your factory automatic. That’s right, six forward speeds at the flick of a switch!
For example, the popular C-4/C6 automatics found in most classic Mustangs have first, second and “Drive” gears. With the addition of a Gear Vendors Under/Overdrive, you would have first, first/overdrive, second, second/overdrive, third and third/overdrive gears to choose from, either manually or electronically via a special Gear Vendors electronic control module.

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But what gear ratios would you actually end up with? Take a look at the accompanying Gear Vendors chart and see for yourself.

Axle Ratio
Trans Ratio
Final Drive Ratio
1st 2.4612.6212.0011.2210.099.599.188.737.94
OD 1.929.849.368.757.877.497.166.816.19
2nd 1.467.497.126.665.995.695.455.184.72
OD 1.145.845.555.194.674.454.254.043.68
OD 0.784.003.813.563.203.042.912.772.52

How does it actually work? To shift the Gear Vendors Under/Overdrive manually, you would pull the shifter into low, accelerate and then hit the floor-mounted control button. A green light will appear, indicating overdrive. You are now in first/overdrive, which is right between first and second gears. (This practice is called “gear splitting”). You would then shift to second with the control button off, bring the revs back up and hit the button again to access second/overdrive. And so on.
Or; should you not want to go through all of this, you can switch the Gear Vendors electronic control module to the automatic setting and let the unit do the work for you. You can also shut the Gear Vendors unit off entirely and let the stock transmission do all the work.
Previously, we mentioned the load-capacity restrictions of the stock Ford AOD and AODE transmissions. Gear Vendors warranties its Under/Overdrive units to handle 1200 horsepower without any modifications, and can set them up to handle as much as 2,000!
The standard units feature a 14-sprag clutch pack, heavy-duty main shaft, needle bearing, planetary-gear setup (which is virtually indestructible) and a CNC-machined, one piece transmission coupler. In other words, a Gear Vendors Under/Overdrive is built virtually bullet-proof.
How much does one of these units cost? Approximately $2,200 to $2,400 for the average classic-Mustang conversion. That’s slightly higher than the standard $2,000 AOD/AODE conversion fee, but you get exactly what you want: reliability, economy and performance.
For your money, you’ll get the transmission, coupler, cast-aluminum transmission-tailshaft adapter, electronic control module, floor-mounted “dimmer-type” foot switch, speedometer-cable converter, all the necessary wiring harnesses and both the short- and long-version installation manuals. And Gear Vendors Under/Overdrive transmissions come with one of the best warranties in the industry: two year for non-commercial use with a 30-day “diver/use” money-back guarantee.
Joe Mangione at Pony Concept Cars recently contacted us about photographing the installation of one of these units beneath Cameron Childs’ candy-apple-red 1966 Mustang GT Convertible.
“The beauty of these units is that you can install them in any Mustang built,” says Mangione. “To perform the conversion, you have to remove the stock C-4/C-6 tailshaft and driveshaft. (Note: the kit does not include a new driveshaft.) Should you ever decide to sell the car, you can remove the Gear Vendors transmission, replace the stock Mustang tailshaft and driveshaft and use (the overdrive) in something else.”
In fact, Gear Vendors has an exchange program wherein they will take your old Under/Overdrive unit in trade or refit it with the tailshaft adapter required by your new application.
Follow along with us as Pony Concept Cars installer Jim Bentley, show you how easy it is to split gears-the Gear Vendors way.

1. Pony Concept Cars technician Jim Bentley begins the installation by loosening the 1/2-inch U-bolts which hold the factory Mustang driveshaft in place.

2. Next, our installer removes the emergency-brake-cable assembly and equalizer bar from the front of the C-4 transmission yoke. These pieces will be reused.

3. From there, Bentley unbolts the 9/16-inch factory transmission mount, then removes the transmission yoke, or crossmember, itself.

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4. Here we see our installer unbolting the four 9/16-inch bolts which hold the stock C-4 tailshaft in place. With the automatic-transmission fluid completely drained, the tailshaft is removed and stored.

5. The upgrade begins with the installation of the Gear Vendors transmission adapter, which bolts up to the C-4 using the same 9/16-inch bolts. Note that a new gasket (provided in the installation kit) is used.

6. Here, Bentley test-fits the one piece, CNC-machined Gear Vendors transmission coupler to the tailshaft of the C-4.

7. In order to achieve proper tailshaft-to-mainshaft clearance, .040 inches of shims were installed at the mainshaft end of the coupler. This is crucial for achieving proper end-coupler-to-main-shaft clearance and eliminating the possibility of leakage.

8. With the new tailshaft adapter and coupler assemblies firmly bolted in place, Bentley re-installs the factory transmission mount and yoke using the factory 9/16-inch bolts.

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9. Shown is the speedometer port on the driver’s side of the planetary transmission. This unit uses its own cable assembly, and the factory C-4 speedometer gear.

10. From there, in goes the Gear Vendors Under/Overdrive unit itself. It’s bolted in place with four 9/16-inch bolts.

11.At this point, Bentley re-installs the emergency-brake equalizer bar. However, it was necessary to bend the stock bracket (located on the transmission yoke) down and back one inch to make it compatible.

12. This procedure is followed with the installation of the speedometer-cable adapter, which uses a special drive pin to interface with the C-4 speedo cable. Once this unit is linked together, it is tie-wrapped from harm’s way.

13. Next, comes the installation of the new driveshaft. On C-4-equipped early Mustangs, a driveshaft length of approximately 36 ½ inches is ideal. But don’t take our word for it. Measure it yourself.

14. Here we see Bentley lubing up the Gear Vendors Under/Overdrive gearbox. This unit uses either GM or Mopar synthetic gear oil. A petroleum-based gear oil can also be used, but will result in a firmer shift.

15. At this juncture, our installer routes the wiring harnesses up through the engine compartment and through the firewall.

16. Power for the foot-activation button is taken from the power source at the sill plate.

16. Using the “Velcro” method, Bentley installs the Gear Vendors switch assembly directly below the headlight switch.

17. The “brain”, or controller assembly, is mounted in the glovebox, with hot lead drawn from the ignition switch. Note that this unit utilizes “telephone-type” electrical hookups, which are simple but effective.

18. Lastly, we see the placement of the foot switch on the floorboard approximately 1 ½ inches from the headlight-dimmer switch.

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