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Maximum Overdrive
Normally when you hear the word, "Over Drive", you immediately think of Mom's Taurus wagon toolin' around town picking up the kids from school and a load of grocery's from the market. Impressions of under-powered, over-geared, low performance family cruisers are envisioned in a horribly uneventful atmosphere.

The primary function of most Over Drives simply multiply the gearing of the normal high gear in a transmission (which is usually 1:1), and
over drives it, or raises the final gear ratio in order to reduce engine rpm's at the same given speed. This is very common with newer vehicles on the road in order to increase fuel mileage while decreasing engine wear and emissions. You may think you don't need an over drive, but then again you may not know that you do.

If you already have a transmission with an over drive, you also know that the difference between the normal drive gear and over drive can be a big "jump" in rpm loss. This leaves you making the choice of either "winding" the engine out in the lower
drive gear, or attempting to "lug" the engine in the higher O.D. gear. The continual "hunting" or sometimes called "seeking" of an automatic transmission by periodically down-shifting and up-shifting, tells you that "Mr. Transmission is not very happy", while manual transmission owners can tell you this to your face. The "Pop-Eye the Sailor" sized right arm can also reaffirm their tale of woe. Installing oversized tires or pulling a trailer will magnify this situation even more.

Those hearty souls not fortunate enough to have a factory over drive transmission see even more difficulty selecting just the right rear-end gear ratio in order to achieve both top performance, as well as maximum fuel efficiency and minimal engine wear. The selection process is difficult at best, impossible at worst.

Factory O.D. Transmissions
Those with the factory O.D. units can spring for new gears, which if in a heavy duty 4x4 can run upwards of $1,500 including gears, bearings, install kits and usually new differential carriers as well as the installation labor- on two axles. The problem is the big 30 percent split between Drive and Over Drive is
still there, but now the O.D. gear is much more needed with no choice of a higher "economy" gear available anymore with a mere shift of a lever.

Non-O.D. Transmissions
Owners of the "overdrive-less" rigs
can make the relatively monumental choice of retrofitting their non-O.D. vehicle with a later model overdrive trans. However, you may need to replace the main ECU computer for your EFI engine with one that is compatible to the new electronic transmission, among other things. Although that may not be the case in some of the pre-90's trucks, nevertheless, the cost of both purchasing and O.D. trans as well as rebuilding one, will cost a pretty penny... say about a hundred and fifty thousand of them. For those not into the math thing, at least 1500 bucks besides the $400 computer purchase to run it may drain your wallet. And always, if installing anything but a brand new unit, have a reputable rebuilder freshen up any used automatic transmission before installation

Other Options
There is also the option of installing an extremely low-geared, low-range transfer case. However, there are draw-backs as some claim with higher loads, larger tires and increased horsepower, that it places undo stress on the upper driveline (driveshafts, yokes and pinion gear shafts). Unfortunately most pinion gear shafts are woefully small to begin with, and not a real likely candidate to withstand the increased torque in all but slow, crawling situations. Gearing at the differential on the other hand, relieves the upper driveline of increased torque stress, and shares it with the ring gear and the much stouter axle shafts. However, you have once again compromised highway economics with off-road formidability. Furthermore, extremely low-geared transfer cases will not help out on the highway in high-range mode. 

The Answer is Near...
So then, now that you're really scared of overdrives, I'm going to let you in on a little secret. Although aftermarket overdrive units have been around for a long time now, there is
another application they can be used for that'll really fill in a niche that we've been looking for here. Say for instance that you have two trucks. One's a newer 4x4 Sport Utility Vehicle, and the other's an older full sized dually pickup truck.

Everyday and Off-Road
The brand new Sport Ute is still under warranty. The stout little motor complete with manual 5 speed trans is peppy, but you want to take this rig on some
real trails, as well as drive this same truck to work 40 miles every day. You'd like to pep up the motor with some upgrades, but worthwhile power goodies are difficult to find besides that warranty thing buggin' you. The wrath of the emission gods striking their mighty wrath down upon eliminates the "other" alternatives. So, you want more power, want to rock crawl on the weekends, have to drive highway speeds to work everyday, and you would like to do most of this work yourself. Oh yeah, and you don't want to spend a small fortune (imagine that... !)

First you opt to have a set of neat low differential gears installed, but again you have that voice that says to 'be careful.' Even though you already have a factory O.D. transmission, you know you'll have to pick a more
reasonable gear like 4.11 over your standard 3.07's or 3.73's. That's fine, but what you really could use for off road would be something in the 4.56 or even the 5.13 range. Now that would get the trail leaders attention when you idle your stock looking SUV straight up a vertical wall! Problem is, a low gear such as that would leave you at the gas pumps more than on the highway when in the everyday transporter mode.

Towing Power and Economy
Alright, one on the board. Now lets quick take a look at the neighbor's big dually pickup. This older truck never came with an over drive, and the 3 speed automatic is nice, but not enough gearing. Being a heavy duty truck, it already came with 4:56 gears from the factory, and spins the V8 pretty good going down the interstate at 65 mph.  Unfortunately, the gears and engine are
needed to tow your 8,000 pound car trailer and 4x4 through the rolling hills to your favorite playground. You need the power, but you don't want the poor fuel economy, increased engine wear and oil consumption, especially when the truck is not participating in it's weekend towing duties.

Here are two neighbors, with two completely different vehicles and two totally different situations. But, both of these scenarios can be solved by the same solution... an aftermarket bolt-on
Overdrive unit!

Surprise, Surprise!
So, you thought that O.D.'s were only for Taurus wagons? In the case of the SUV, an auxiliary over drive unit could allow the use of ultra-low gears to take every advantage while off-road, yet allow the factory O.D. coupled up with an aftermarket O.D.'s additional increased gearing, to keep the fuel economy, reliability and engine wear exactly where it was from the factory, if not
better! No expensive and warranty canceling engine mods needed, with no sacrifices on the highway at all.

Our neighbor with the big dually pickup and heavy duty towing gears can still yank that trailer around the hills and valley's with ease, and will also be able to cruise the flat lands in comfort and maximum economy... on the same trip! With the auxiliary overdrive engaged, the coolant and oil temperatures will be lower, including the engine, transmission and even the transfer case. With older, as well as bigger cubic inch engines, the oil consumption will be much less with a lower revving motor, too.

Let's Shift Into Overdrive...
Gear Vendors, Inc. out of El Cajon, California market a product called an Under/Overdrive, which is a true bolt-on auxiliary transmission. The unit can be easily installed on the rear of transfer case or transmission by replacing the existing tail shaft housing with only one drive shaft needing to be modified. Realistically, you really don't need the advantages of the overdrive gearing when engaged in four wheel drive... unless you plan on cruising the highways locked in at 70 mph plus. So why go to all the work of splitting the transfercase, modifying two new drive shafts, relocating new cross members, relocating shift handles and more. Only the full time and all-wheel drive models will require this extra step.

Built For Heavy Use
The U/OD unit can't get stuck in neutral between gear changes like the countershaft design can, otherwise requiring to come to a complete stop to 'reset' the unit. A Pressurized lubrication system, rather than merely splashing oil around by the gear action, extends the life of the unit as well as minimizes power loss. A Permanent filter insures clean, reliable operation, requiring fluid changes only when periodically changing the main transmission's fluid as normal maintenance.


Once installed, the operation is as comfortable and familiar as shifting your factory transmission. A computer box supplied with the U/OD unit can be set to "manual", which provides standard operation while allowing the driver to shift into overdrive at any time, in any gear. That's right, just like a
gear splitter , or 2-speed axle found on the big rigs.

Your 3 speed automatic is now a 6 speed. Your factory overdrive transmission is now an 8 speed automatic. And of Course your 5-speed manual is a very formidable 9 speed manual, but with a real neat bonus... with the Under/Overdrive unit, you now can upshift or downshift from direct drive to overdrive
without depressing the clutch or letting off the gas! You can enjoy all the advantages of a manual transmission, and have the benefits from an automatic overdrive as well. Better yet, the computer control box can be set to "automatic", allowing hands-free operation of the overdrive at higher speeds, while utilizing the lower gearing and more efficient operation at lower speeds... all by itself. In auto-mode, the over drive engages at around 47 mph and disengages as low as 18 mph.

The Choices are Endless
Used as an overdrive, you can have the low gears needed for power, and the higher gearing for economy. Using the U/OD unit as a gear splitter, you can select exactly the gear you need, by using the available ratios
between the gear selections on your stock transmission. This would be ideal for narrow power banded diesel engines. Other good choices would include hi-revving and power hungry motorhomes and dump trucks as well as allow low geared off road trucks to switch to smaller "every-day" tires while retaining their driveability at highway speeds. Besides being easily installed, it can also be just as easily removed when selling the vehicle, if so desired. But I hardly think the new owner would want to let go of it.

And the Check, Please...
The average cost of the Under/Overdrive unit and driveshaft modifications run around $2,500. This is certainly not a low dollar venture, but then again, you're not looking for cheap and the inherently un-reliable equipment you'd get for your money in
this department, would you? The many benefits in operation, fuel savings and reduced component wear will justify your purchase in a short while, if not immediately.

A Word of Caution...

Gear Vendors told us that during very steep down hill runs, the under/overdrive unit should be disengaged to both assist in slowing the vehicle as well as take undo strain off the over-running clutch. Although the unit is built to withstand this additional stress, that would be the weakest part of the unit's design. The overdrive function will also not work in reverse, and we don't see any reason to want it too, either. There is also a built in safety feature to keep the overdrive from actuating in 4 wheel drive that could otherwise cause driveline damage.