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Double The Gears

Installing The Gear Vendors Under/Overdrive In A Tri-Five

Grant Peterson Sep 19, 2005

In the pursuit of building a cool car today, there are many avenues to consider taking. Why not go down whichever path you pick with three more gears to choose from? Unless you have been in the far reaches of our solar system, you have probably heard of Gear Vendors. By now, and as far as I can tell, they have turned the way we think of transmissions upside down. Mark Newberry, the owner of this '55, wanted to explore Gear Vendors' ideas in the pursuit of some quick numbers at the track as well as reliability and economy on the road. Mark's car has a small-block GM crate motor and a TH400 tranny and as we found out, everything is pretty much a bolt-in job for the Tri-Fives except for drilling the holes in the dash for the indicator lights. You may also need to put a kink in an E-brake bracket, even if the tranny crossmember/ mount stays in the same place. Other than that, all you need to worry about is wiring.

Once you have an idea of what Gear Vendors can do for your car, you may be in a state of euphoria. Rick Johnson, owner of Gear Vendors, believes the strongest transmissions for performance cars in our range of the hobby are the TH400, TH350, Powerglide, and M21 or M22 Muncies. I am told that the reason the early trannies are stronger is that GM cared about strength then, and today's gearboxes are all about things like fuel costs and EPA legislation. Gear Vendors says "Why not take out your musclecar-era three-speed auto or four-speed manual and replace it with a late-model overdrive trans that is inherently weaker when you can keep the superior strength and add a superior overdrive to it." GM did exactly that with the Callaway Twin Turbo Corvettes, which only came as automatics with a Gear Vendors/TH400 combination instead of the factories own 700-R4 or 4L80E. Ratio is the big key to the success that Gear Vendors enjoys. Gear Vendors has the same ratio spread as a 2-3 or 3-4 shift of a Muncie close-ratio manual, which is also the same ratio that many performance gearboxes have used (i.e. Porsche, Ferrari, BMW, etc).

With this ratio, Gear Vendors provides the automatic in your Chevy with twice as many gears and you guys with three pedals get some cool features like clutchless shifts, for example; check the Gear Vendors Web site for full details. Virtually all three-speed automatics have the same ratios within a few hundredths (2.5 First, 1.5 Second, 1.0 High). This means your transmission takes 2.5 engine revs to turn the driveshaft once in First gear, 1.5 in Second, and is direct drive 1:1 in High gear. Gears are multipliers of torque. Putting a 2.0 gear between First and Second is going to keep both torque multiplication and engine rpm high which means more hp getting to the rear wheels. If they can then put a 1.2 gear between Second and Third, and an overdrive gear beyond Third. Then we have a close ratio six-speed overdrive trans with four nderdrive power gears, a direct-drive High gear, and an overdrive Cruise gear, hence the under/overdrive moniker. Let's top all this off with the improved economy, expect 25 percent thereabouts cruising with a normal aspirated engine, thanks to 22-percent lower rpm. With the overdrive, a 3.73 rear gear will feel just like 2.91 when cruising! With the way gas prices are headed, the money saved on gas will pay for the unit and beyond. That's the layman's explanation of the Gear Vendors principals, or you can just call it the best of both worlds!

There are two styles of electronics you can get from Gear Vendors: the standard Auto Drive package and the optional, new AutoShift sequencer. Late last year at the SEMA show, Gear Vendors debuted their new AutoShift electronics, which garnered quite of bit of press. Since it was something new, we thought we would try them out on this install just before they are being made available to the public.

The difference to the standard package is that the AutoShift takes those shifts where you are moving the shift lever and clicking the Gear Vendors and automates the process. So, launch in First and immediately press the button for First-over (like the standard electronics the overdrive will shift to First-over at the appropriate speed--when you are tached up and ready for Second, move the shift lever and when the trans grabs Second-gear the AutoShift sequencer will take out the overdrive without you having to click the button. When you are ready for Second-over just click the button, and when you move to Third the sequencer takes out the overdrive. When you are cruising and click in the overdrive, it will turn itself off at low speed/rpm and when you want it back on just click it in. So you always tell it when you want to shift in to overdrive in any gear which preserves the fun interactive part and still allows you to decide on 1/2 steps in gearing or full steps depending on what mood, acceleration, or who's watching. The sequencer just automates the deactivation of the overdrive. Well, that's a good amount to absorb for now, stay tuned for more on Gear Vendors in the future.

Let's take a trip to Firehouse Fabrication in Placentia, California, and follow owner Brett Maxwell as he walks us through the Gear Vendors install on this '55 that is nearing the first stages of completion.


The mechanical parts from Gear Vendors look pretty simple, but don't let that fool you. There has been some serious R&D that has gone into their revolutionary product. We have the over/underdrive unit, trans to Gear Vendors adapter, yoke, speedo cable adapter and extension, output shaft coupler, floor mount switch, and miscellaneous parts.

This shows the electrical side of things with the AutoShift Sequencer box and all the needed wiring that is labeled extremely well and comes with all the correct connectors crimped on already.

First remove the driveshaft, place a bucket underneath to catch any leaks. Remove the two bolts that go through the crossmember and in to the tranny mount. Jack up the transmission to take the weight off the tranny mount and remove the tailshaft housing from the tranny.

Clean the mating surface on the stock trans where the tailshaft housing was removed. Lube the bushing and seal in the Gear Vendors adapter with white lithium grease. Place either the O-ring or paper gasket (depending on your tranny) on the adapter dry, no silicone, and bolt it up to the tranny. Make sure to torque the bolts to 40-45 lb-ft. You can also bolt the rubber trans mount onto the adapter and lower the tranny back onto the crossmember.

Next, Brett taps on the output shaft with a soft face hammer to make sure it is seated in the tranny properly.

Put a light coat of white grease on the splines of the coupler and slide it onto the output shaft. Put a straightedge across the face of the adapter and using the supplied round metal shims, stack them up under the straightedge until they fill the gap between the coupler and the straightedge. Remove the coupler and put that stack of shims inside the coupler perpendicular to the splines and put the coupler back on the output shaft making sure the shims didn't fall over in the process.

Now you should be able to put the straightedge across the face of the adapter again and using a feeler gauge, measure a gap from 0.00-0.015 inches. If you need to remove or add more shims to get the right gap, do it now.

With the printed side of the gasket facing the Gear Vendors Overdrive, slide it in the studs dry, without using silicone again and bolt the unit to the adapter. Tighten the eight nuts in a criss-cross pattern to 6-9 lb-ft...

...Take your time; they are kinda tough to get to. You can also install the yoke at this point. Make sure and get a bottle of GM#12346190 synthetic gear oil from either a dealer or good parts house, lube the splines and the seal and insert the yoke.

Cap the old speedo inlet with the supplied parts.

Lube with white grease and install the speedo gear into the new housing and place the new O-ring in the groove on the housing. Lube the whole assembly and slide it into the overdrive unit making sure it is seated properly and tighten the retainer fork down over it, taking care to line up the slots with the fork.

Since we have now lengthened the trans and pushed the speedo gear farther back, we need to lengthen the speedo cable. The speedo adapter is also a signal generator for using electronic speedos. This car still has a mechanical speedo, so the wires won't be used, but we still need it to attach the extra length of speedo cable to. Brett used an Adel clamp mounted to the floor to secure the speedo adapter and cable.

You can see in this picture that this emergency brake bracket is very close to the yoke, so Brett and I figured if we put a jog in it everything would clear.

Mark where the bend needs to start and finish so it doesn't bind with anything.

Making sure we remembered where each side needed to be bent, we put it in the vise and persuaded it with a big hammer. After the two bends were done, you still want each end to be parallel.


It's time to measure for a new driveshaft, or in many cases, get it shortened. An easy way to do this is to seat the yoke all the way into the tranny and measure from center to center of where the U-joint cups would be and subtract 5/8 inch. That will leave just enough travel in the driveshaft and minimal yoke sticking out of the tranny. Eliminating extra travel ensures more spline overlap and makes for more driveline stability.

I took a trip out to Inland Empire Drivelines to get the new driveshaft made and Mario got right to work once I gave him the measurements. There are many options these days in driveshafts, but the customer opted for traditional steel.

Mario's all smiles over this beauty! In the process he installed new Spicer U-joints. The front is a 5-799X and the rear is a 5-785X to mate up to the 12-Bolt rearend, he also made sure we had the straps for the rear, which is a 2-70-28X, but this differs with the style and year of your rearend.

To use the AutoShift Sequencer with a non-electronic transmission we need to mount a range sensor to the tranny's shift arm and have that signal sent to a control module. In order to do that, we installed Lokar's Indicator Control Module. Lokar's was chosen because it can drive a gear position display (PRND12) if needed later.

Lokar's installation instructions are very simple and it shouldn't take long to install if followed correctly.

We are only going to need the orange and brown wires from the Lokar harness; the others can be capped off. The two wires will go directly into the AutoShift Sequencer box, easy enough.

The lead wires from the solenoid are run up along with the Lokar harness.

For the tach lead, we pull it from the tach connection on the HEI cap directly to the control box.

After much thought we had decided to mount the AutoShift Sequencer box on the firewall due to the lack of space under the dash in this car.

Take some time and decide where you want to mount the overdrive indicator lights. We tried to incorporate them into the dash as much as possible. Lay them out together if you can so it's easy to see both of them at the same time. You will also be happier in the end if you take the time to measure out their placement before drilling holes in your dash.

Mount the foot switch where it is out of the way, but still easily accessible and comfortable to use.

The wiring is very basic and simple for this unit, everything is labeled well and the directions are easy to read. I would only make it more difficult if I tried to rehash them.

You can't see it very well, but there is a hole drilled in the firewall with a grommet in it behind the wiring to the right of the Auto Sequencer. This was used to bring the wires from the indicator lights and the foot switch to the box. That's it, cruise like you've never cruised before!


Inland Empire Driveline
Ontario, CA 91761
Gear Vendors
El Cajon, CA 92020
Lokar Performance Products
Knoxville, TN 37932
Firehouse Fabrication
Placentia, CA 92870

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