No one builds a hot rod to save gas, but downgrading to bare essentials and stiffing your roommate on rent will only get you so far before it’s time to park that 10-mpg big-block. Even when gas was still cheap several decades ago, Gear Vendors recognized these issues, and in response, engineered a bolt-on overdrive unit. These slick gizmos attach to the back of just about any transmission, and add an overdrive gear that can be actuated from inside the cabin with the flick of a switch. The really cool part, however, is the ratio-splitting ability offered by a Gear Vendors unit, which adds a host of underdrive ratios to the mix. This lets you keep the motor in the fat part of the powerband, which explains why Gear Vendors units have become very popular in hard-core racing applications. That’s no joke, as Gear Vendors overdrives have been popping up in 6- and 7-second drag cars in recent years. To learn more about everything these trick overdrive units have to offer, we called up Rick Johnson at Gear Vendors to solicit his expertise. Here’s what he had to say:
With gas prices spiking once again, many hot rodders are contemplating upgrading to an overdrive transmission. A Gear Vendors overdrive unit lowers engine rpm by just over 22 percent by utilizing a 0.778:1 overdrive ratio. This means an engine that is turning 3,500 rpm at cruising speed would see a 777-rpm reduction in engine speed. At 4,000 rpm, the reduction increases to 888. Interestingly, since the mathematical reciprocal of 0.778 is 1.286, maintaining the same rpm after turning on the overdrive unit would represent a 28.6 percent increase in velocity. For example, if you were going 65 mph and turned on the overdrive while holding the same engine speed, the vehicle would go 84 mph.
Overdrive uses the torque of the engine to more efficiently propel the vehicle, and is the single biggest improvement in mileage you can achieve with a performance part. Typically, a naturally aspirated engine with a carburetor will achieve close to a 28.6 percent improvement in fuel mileage with a 3.42:1-or-higher (numerically) ring-and-pinion ratio. If you have 4.10:1 rear gear, for example, the car will cruise just as efficiently and with the same rpm as if you had 3.19:1 gears when the overdrive is engaged. Forced-induction vehicles will get an even greater improvement. A small roots or centrifugal supercharged car will likely get a 50 percent improvement, and a big 8-71 blower will typically see a 100 percent improvement because you are slowing down the blower and taking advantage of large torque values.
How it Works
A Gear Vendors overdrive unit is simply a single planetary gear combined with a direct drive. There are 77 teeth cut into the drop forged annulus ring gear, and 22 teeth on the sun gear, which is attached to a common vertex cone clutch. In direct drive, all forward force is carried by an 18-element Cooper roller clutch that can handle over 2,500 hp. When the solenoid closes the pressure circuit, the clutch is pulled against a brake ring with 700 psi of pressure, which stops the sun gear. With the sun gear stopped, the planet gear must advance the ring gear by 22 of its 77 total teeth. So if you were to look inside the overdrive at speed, it would seem to be turning about one-third of the speed of the driveshaft.
Part of the appeal of a Gear Vendors overdrive unit is that in addition to using it as a top gear, you can also control it manually in any gear. This is an important benefit as drivers enjoy more gears for the autocross or twisty roads, where having closer ratios improves car control and fun factor. Internally, there is a plunger pump driven by the input shaft of the Gear Vendors overdrive. A relief valve keeps residual pressure at 30 psi in the system, and within a few pump strokes, that pressure increases to 700 psi when an integral 12V solenoid closes the relief valve. So, in-shifts are directly related to driveshaft speed, which allows for some delay in First gear, a very slight delay in Second gear, and then a near instant shift in Third gear. Out-shifts of the overdrive are achieved when the internal pressure inside the system is dumped, resulting in instantaneous shifts. This makes the ratio-splitting timing easy for the driver.
All the newest factory automatic transmissions of five or more speeds operate on this same principle. They have more than one overdrive gear and two underdrive ratios, and then just turn on different combinations for each gear. This way, four planetaries can yield up to nine ratio. Yes, Gear Vendors did all this first, but now 20 years later, ratio splitting with automatics and dual-clutch manuals is becoming more common at the OE level.
Reducing Parasitic Loss
When designing transmissions and overdrive units, durability and reliability are often at odds with minimizing noise and parasitic power loss. Gear Vendors employs several techniques to reach a happy medium between these design goals. This is evidenced by the fact that our overdrive units require just 1 hp to turn for every 400 hp put through them. A large part of this efficiency is due to the Gear Vendors common vertex clutch design. This clutch is a single member, meaning that it shifts with very high pressure compared to the clutch pack in a conventional automatic transmission. The advantage of a clutch pack is that you can achieve a wider ratio spread, and slowly bring in ratio changes for smooth shifts that grandma would like. The single-element clutch is ideal for a short ratio. It results in a firm engagement, which is why a Gear Vendors unit doesn’t need a separate fluid cooler and operates on just 3/4 quart of ATF. Since a Gear Vendors unit does not share the same fluid as the rest of the transmission, if you break the trans, it will not hurt your Gear Vendors one. Ultra-low parasitic loss is the key to long life, and Gear Vendors–equipped off-road race trucks have survived 7,500-mile events with zero reliability issues.
Gear Vendors’ trademark Under/Overdrive feature refers to the overdrive unit’s ability to also give your transmission twice as many underdrive ratios. In a typical three-speed like a TH350 or TH400, adding a Gear Vendors means that a vehicle now has four underdrive gears, one direct-drive gear, and one overdrive gear. Furthermore, if the vehicle has a 3.55:1 rearend gear, when the driver hits overdrive while in First gear, he has more torque multiplication than if he shifted into Second gear with a 4.56:1 rearend gear. That results in some serious bottom-end power. While a wide ratio late-model trans can certainly turn the tires over, when it shifts to Second gear you get a tank slap, as the car hooks hard due to the big rpm drop. Older muscle car–era transmissions don’t have this problem because of their closer ratios. Likewise, Second gear in many automatic transmissions is too low to use as a passing gear, resulting in too many rpm. Hitting overdrive in Second gear with a Gear Vendors gives you much better gearing for passing.
In road racing or on the autocross, short ratios are a must when you want to exit corners as quickly as possible. Many racers buy Gear Vendors overdrives to exploit these areas of performance, and they also appreciate the general driving feel of having just the right gear for engine braking and acceleration. We also have some very successful drag racers running Gear Vendors units as well. Using the Under/Overdrive function in drag cars helps reduce rpm drop between shifts, and keeps engines in the fat part of the powerband.
Reduced Driveline Stress
In addition to the performance and fuel mileage benefits of a Gear Vendors overdrive unit, it also reduces the stress placed on the transmission and driveshaft. Shorter steps in gearing means the transmission has to accommodate less of a speed differential in its clutch pack and in the force applied to the gearing. The primary problem with wide-ratio, late-model transmissions is that the ratios themselves are harder on the transmission. A vehicle can’t instantly go 50 percent faster on a change from one ratio to the next, so the load must be absorbed by the transmission, torque converter, driveshaft, and engine.
GM TH350 or TH400 transmissions, with their closer ratios, are therefore inherently stronger than their late-model overdrive cousins, like the 4L60E, even if the gears themselves aren’t any stronger. After the OEs became subject to the CAFE rules imposed by the EPA in 1981, The Big Three had to get every last tenth of a mile per gallon they could out of an EPA test cycle. By lowering First gear to 3.06:1 in the 700-R4, and then upshifting to tall engine-lugging ratios while locking up the torque converter, they could get those extra tenths of a mile per gallon. The issue for hot rodders is that we don’t care about tenths of a mile per gallon, we care about performance and durability.
Consequently, the late-model transmission will simply never be as strong or as capable as a muscle car–era design. By using a Gear Vendors overdrive, you can increase driveshaft speed 28.6 percent relative to the gear you would be in without overdrive, and then shift up just half a gear. This greatly reduces the load and work the transmission endures to step into the next ratio. Also, since installing a Gear Vendors unit requires shortening up the driveshaft, the driveshaft becomes shorter and stronger as well.
Gear Vendors overdrive units fit a wide variety of vehicles, and installing one is very simple. While first-gen Novas require modification to the tunnel, virtually all other applications only required shortening the driveshaft, replacing the transmission extension housing with the Gear Vendors housing, and then bolting on the overdrive unit itself. Everything is included except the shorter driveshaft, and it can be installed in most cars in just three to four hours. Not surprisingly, Gear Vendors’ most popular installations are on the big-block–era cars where the owners prefer to keep their stronger muscle car era transmission instead of swapping it out for a late-model overdrive transmission. Gear Vendors units were factory equipment in Callaway’s twin-turbo Corvettes, which used TH400 transmissions instead of 700-R4s. Since these turbo motors produced gobs of torque and horsepower, Callaway felt they were better suited to a TH400 with a Gear Vendors.
Simply put, a Gear Vendors is the strongest overdrive unit ever made for cars and trucks in racing applications. Our standard product can handle over 1,250 hp and carries a two-year warranty that remains valid even if used in racing applications. Our $350 Pro Mod upgrade increases horsepower capacity to 2,500. There were a half-dozen 6- and 7-second cars equipped with Gear Vendors units that competed in Hot Rod’s 2012 Drag Week competition. We have customers who go through several TH400s in a season, but have been running the same Gear Vendors unit for over a decade. There is no other overdrive guaranteed to even half this horsepower level, or that offers even half the warranty period when used in race cars. At Gear Vendors, we expect our product to outlive your car. With a price of $2,695 for the typical Gear Vendors kit, they aren’t cheap, but the overdrive will pay for itself in the long run and adds value to the car.
Factory vs. Gear Vendors Overdrive
In the automotive aftermarket, companies often manufacture 700-R4 or 4L60 transmissions that say they’re good for 600 hp, but oftentimes they will not guarantee it if you race with it. No builder wants you to jam 600 hp into that overdrive at full throttle. And read the warranty. In most instances, you will have to send it back to the manufacturer at your expense and wait while they rebuild. This is why Gear Vendors recommends buying an overdrive transmission rated for at least double your horsepower. That’s not to say that there aren’t a ton of people happy with their 700-R4 because most of the people who buy 700-R4 and 4L60E automatics do not abuse them. Nevertheless, Gear Vendors would argue that no performance car should have a First gear ratio lower than 2.52:1 since it makes all the other ratios too wide, ruining both reliability and performance. Even though Gear Vendors overdrive units are known for their extreme horsepower capacity, 13- and 14-second cars have the most to gain—from a performance and fun factor standpoint—through ratio splitting.
Bolting a Gear Vendors unit on a factory Muncie four-speed manual trans may seem unnecessarily complicated when there are many aftermarket five- and six-speeds available. However, adding a Gear Vendors to a Muncie offers several advantages. Tremec and Borg-Warner both build fine products, but you can’t powershift these transmissions because their ratios are too far apart, resulting in too much stress on the synchros. With tighter ratios of a Muncie, this simply isn’t a problem. Another advantage of the Gear Vendors unit is that in most applications, there are no tunnel or crossmember modifications that are required.
A Muncie, or even a Top Loader or Mopar A833 for that matter, is bulletproof and has between a 2.20- to 2.52:1 First gear. This keeps the other gears’ ratios close for both upshifts and downshifts. The ratio spread of a Tremec TKO 500—between First and Fifth gear—is 80 percent, whereas a Muncie with a Gear Vendors unit is 65 percent. Gear Vendors also makes kits for the G-Force GF5R, Richmond Street Race five-speed, Jerico DR4, and Tex Racing road-race four-speeds.
In order to build overdrive units capable of handling 2,500 hp, Gear Vendors uses only premium materials to maximize durability. Input shafts are built from 300M steel. They start out as annulus forgings of a special British specification that arrive as 12-pound raw blanks that are then machined down to just 4 pounds. The all-steel planet carrier in a Gear Vendors unit is a very costly design that has twin rows of high-speed cage needles in each gear. They can run 250,000 miles and still look new. Gear cutting is performed on our Mitsubishi GE15 hobber, and the crowns are finished on a Hurth shaver. By constantly reinvesting in state-of-the-art machinery for the last 32 years, Gear Vendors has been able to achieve an incredible level of quality control in our overdrive units.
Off-road racing trucks equipped with Gear Vendors units have won every FIA cross-country rally event except the Dakar Rally. This includes wins at the Qatar Rally, the 24 Hours of France, and the Tunisian Rally. The trophy trucks in these races produce 950 hp, and compete in races that last up to 7,500 miles. Gear Vendors–equipped trucks have also won national titles in CORR off-road racing. Likewise, Gear Vendors has posted more than 20 wins in various land speed racing events at Bonneville, Maxton, and the Texas Mile. In recent years, Gear Vendors has had a lot of success in Hot Rod magazine’s Drag Week competition. In 2012, we won every power adder class except Unlimited. We also posted the second-quickest e.t. in Drag Week history with a 6.942-second pass as well as posting the fastest mph ever at 212 mph in Jeff Lutz’s ’57 Chevy. CHP