Reprint from Classic Chevelle Magazine, Annual 2006



o split or not to split? That is the question. What was gear splitting and what did it do? To be completely honest here, I’m a bit of a Forrest Gump when it comes to gears, gears rations, and the math theories involved with gears. So, you can bolt on an overdrive to your non-overdrive tranny? I get that.
What I didn’t understand was why anyone would want to do that since these Gear Vendors units were more expensive, or at least pretty close to the cost of a whole overdrive transmission—manual or auto. It was time to take a closer look at what this was all about.
Gear Vendors has the idea to make an overdrive that handles extreme or regular around-the-town horsepower with the lowest parasitic losses and can be bolted on the world’s strongest transmissions to make them even better. You probably already own the other half, a three-speed auto or four-speed manual or automatic built in the musclecar-era when 427 cubes were available in a family car.
Let’s apply this to the Chevelle. Gear Vendors (and others I spoke to) believes that the strongest transmissions for the Chevelle are the TH400, TH350, Powerglide, and M21 or M22 Muncies. Gear Vendors says “Why 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 factory’s own 700-R4 or 4L80E. The advantage of adding a Gear Vendors unit to your Chevelle is that it will be able to handle anything you have under the hood, or anything you may add later. The overdrive unit fits the car with the original crossmember and tunnel (through slightly modified). It has been reported that we can never wear it out or break it. The unit is guaranteed for two years, and is unlimited to horsepower, abuse, or daily dragstrip use. That IS cool!

Besides all that, there is added better economy (expect 25 percent thereabouts cruising), lower rpm by 22 percent so the 3.73 differential will feel just like 2.91 when cruising. And you can still keep your period-correct transmission too. I am told that the reason that the early GM trannies are stronger is that GM only cared about strength back then; today’s trannies are all about fuel costs, EPA legislation, production costs, etc. Economy would be nothing without performance right? Gear Vendors did not just grab a ratio from air and set out to make the world’s most bulletproof overdrive. The ratio was one of the keys to success-22 percent or 0.78:1 with the reciprocal of 28.6 percent or 1:1.286. All this math means that the Gear Vendors unit has the same ratio spread as a 2-3 or 3-4 shift of a Muncie close-ratio manual. Many other performance ratio manuals have used this ratio spread and it is the same amount of overdrive found in many performance design manual trannies


A Gear Vendors unit mounted to a display TH350 transmission illustrates how the tailhousing of the main trans is replaced with the new supplied housing and then the overdrive bolts right up. More than half of all Chevelles came with the TH350. Though it handles a little less horsepower than the 400, it is also lower in parasitic loss and still a good trans behind normally aspirated big-blocks. It is much more capable than a 700-R4 because of its closer-ratio spread and combined with a Gear Vendors nets six forward gears.

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This is Rick’s Chevelle. Once the unit is installed you can see what it looks like. Before the Gear Vendors install, measure the driveshaft while it is still in the vehicle and as per the instructions send it with the new Dana yoke that comes with the kit to the local driveline shop to be shortened and rebalanced. If you want to use the factory crossmember unmodified all you need to do is round out the tunnel; this is most easily done by renting an electric or air hammer that has a 1-inch head with a slight roundness like a kitchen spoon. You can use the peening end of a ball-peen hammer or you can use a Gear Vendors crossmember.

(i.e. Porsche, Ferrari, BMW, etc.) With this ratio, Gear Vendors brings twice as many gears in the automatic of your Chevelle (you manual trans guys get some cool features to which is covered later in this article). 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 multiplier of torque and since the math of horsepower is Torque x rpm / 5,252 = hp then putting a 2.0 gear between First and Second is going to keep the torque multiplication and engine rpm both high, which means more horsepower getting to the rear wheels. If we can then put a 1.2 gear between Second and Third. An overdrive gear beyond Third we have a close ratio six-sped overdrive trans with four underdrive power gears, a direct drive high gear and an overdrive cruise gear. Thus. Gear Vendor’s trademark Under/Overdrive is a type of overdrive that can be used to double the underdrive performance ratios and provide overdrive cruising economies.
What about the “seat-of-the-pants” feel? I called Rick Johnson, President of Gear Vendors, and he showed up at my house two days later with a redy-to-rumble, 502-cubed ’70 convertible. Rick is a real car guy who digs Chevelles and brought customer/owner Kevin Watkin’s monster machine over to drive. Rick brings me a car I could definitely get in trouble with and…lets me drive. The Gear Vendors is controlled electrically so it is just a button you can put anywhere from the shifter (like a B&N T-handle with a button in the side or Hurst ball with button in the top) or on the console, or-even on the floor like a dimmer switch. This particular Chevelle was a restored stock interior and meant to appear super-clean. His button was the dimmer switch-style so it looked all factory stock. Small red and green lights were placed in the dash and also looked factory. The red light is used


If you’re curious about how the whole enchilada looks like, here it is in all its gear-splitting glory! This is what comes right out of the box.


All the wiring is pre-harnessed with plug-in connectors for the computer and the appropriate ends for the solenoid, speed signal, power, ground, control light, and dash switch. Fuse taps, wire ties, screws, everything you should need for a simple plug-in installation.


The overdrive comes with a computer that has the features of Auto Launch for ripping through the gears and Auto Drive for complete automated factory-type overdrive cruising. Since the factory horseshoe shifter was used in this Chevelle the owner opted for the dimmer switch control when in the manual position. LED bulbs were installed, and the rocker switch hidden. All sorts of switching options are available from Gear Vendors.

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for the auto features that come with the Gear Vendors electronics (Auto Launch and Auto Drive). The green light is used to display when the overdrive is actually engaged in any gear so that you always know what gear you are in.
You can put the supplied dash switch into the auto mode and the car acts just like it came with a factory four-speed auto with overdrive. First, Second, Third, and then at 47 mph, it shifts to overdrive without the driver realizing it. Then it shifts back off at lower speeds and shows the red light, which indicates it will up shift into overdrive when your speed is higher. Very nice for cruising and you can leave it in this position and never touch a thing.
Put the dash switch in manual and the fun really takes off. Auto Launch means we can select the overdrive at the light before or right when we hit the gas, and the overdrive will shift as we are accelerating and First gear begins to wind out. This lets us keep both hands on the steering wheel while the car accelerates through First and First-over so we are 28 percent farther down the track or road before we need to move the shifter up to Second. When we do move the shift to Second we can either leave the overdrive engaged so we have the factory gear spread or hit the Gear Vendors button right when Second grabs and gear-split Second like we did at First. This was powerful and definitely interactive-like having a manual trans with no clutch so it really steps up the fun factor and keeps the engine right up on pipe in the sweet rpm. Even when you are just at half throttle, the car sounds so much more powerful and fast. I could have driven it until I needed food or water. We took some winding back roads and you could also see how fun this would be when carving through the canyons. Second-over in the straightaways, click the button and you go back down to Second for the corners, hit the apex, accelerate, and grab the button for Second-over again. Something you could never do with a 700-R4 overdrive trans swap because the 700 has even wider ratios then the TH350 or 400. With a Gear Vendors you have M21 ratios just 22 percent different from one another. In fact, and M21 or even a six-speed late-model manual still has just three underdrive ratios: First, Second, Third, where Kevin’s Chevelle has four underdrive gears-two factory and two more in combination with the Gear Vendors.


Take a look at this cutaway and let’s consider for a moment the mathematics involved in its operation. To understand the performance benefits of Gear Vendors, look at these ratios for our example Chevelle with a standard 400 trans and 3.73 rear diff. Gear. Transmission ratio x diff ration = torque multiplication. With the close-ratios you can see why this car sounds like an M21 manual with two more gears and no clutch yet behaves like a factory four-speed automatic when the dash switch is in the Auto Drive mode.

It’s these underdrive gears where all the fun of driving and accelerating is for any of us not on the German Autobahn (or in the state of Montana with it’s un-posted speed limits).
So why don’t the factories make this on the assembly line? The factories could do this but it would be cost prohibitive because Gear Vendors goes to extremes on types of materials and design that are all abut this niche market of big horsepower and performance. For a Chevelle with a TH400 or TH350 the Gear Vendors kit is $2,395.00 plus $45.00 shipping in the U.S. If like Kevin (the owner of the Chevelle we drove), and you do not want to touch your tunnel with a ball-peen hammer, then you will either modify your crossmember to lower the transmission mouth by a ½ inch or order the exchange crossmember from Gear Vendors that is already modified for $100.00. If you have a manual M21 or M22 the kit is $2,595.00 and requires the same mod to the cross member, your manual will get the same 28.6-percent faster overdrive ratio and allow you to shift clutchless to the overdrive in any gear so that you can have some of the same performance as the auto guys. With Auto Launch the M21 car will leave the line in First and shift automatically and clutchless to First-over which is only 4/100ths different than if you shifted to Second and then you can further exploit the time slip by shifting to Third and then clutchless to Third-over which is only 1/100th different than Fourth and do the entire dragstrip run with just one clutch depression.


FIRST – 9.25 Your existing launch gear with a 3.73 diff and 2.48 First gear multiplies torque 9.25:1 over 7.22 This is the same torque as a 4.88 diff car would have in Second gear-super powerful.
SECOND – 5.52 Now Second gear has much more hp and half the engine rpm drop when coming from the 7.22 split over 4.31. Right when the aerodynamics start loading the car you now retain multiplication of a 4.31 ration.
THIRD – 3.73 Low -speed cruising gear is the same a before installing the Gear Vendors.
OVER 2.91 High speed cruising just like having a 2.91 diff. And no overdrive – 25 percent better mpg.

Gear Vendors informed us that the consumer at home installs most of their products. But if having it installed by a shop is your choice, there are nearly 800 authorized dealer in the U.S. that will do these installs if you prefer. On the Chevelle, it is a four-hour installation by those with experience. If you do it yourself, you are probably looking at a couple extra hours, as you will spend time reading and double-checking things. Rick says that a good gauge on the difficulty meter would be that if you can replace your own head gasket that this would be a job you could do at home. If on the other hand you would rather pay someone, call Gear Vendors and they will book you into your local installer.