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:

MPG Improvement

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.