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Auxiliary Transmissions

Bill S

Can anyone provide pro/con information on the product. I am in the process of purchasing one of these units for my Mountain Aire Class A and would greatly appreciate any insider information. Thank You,

Tony H

Bill, either will do the job. Both are tough and well-proven. However, both require driver attention to shift smoothly.

I have both units and I have some additional observations. They're a little too long to post here, but if you will e-mail me your address, I'll send them to you (or anyone else for that matter).

Just remember that advice is worth exactly what you pay for it! (grin)

The information below is courtesy of Tony H.
Thanks for sharing with all of us!

I've been interested in auxiliary transmissions for several years and have researched the two major units currently on the market, namely the Gear Vendors and the US Gear Dual Range. I currently have a Gear Vendors overdrive in my big block powered Suburban which I use for towing. I have had a Dual Range underdrive in the past. The following comments are strictly my own opinions that are based on articles I've collected, brochures, the manuals I have for both, and - in the case of Gear Vendors - discussions with their technical support staff.

Please, use the following comments as a data point only. I’m not specifically recommending either one.

Tony H.
Monroe WA


• Both are husky (up to 30,000 lbs+ CGVW - they are in fact probably stronger than the current factory overdrives used in late model automatic transmissions). Both are well proven units.

•Both provide similar overdrive ratios (GV=22% USG=25%)

•Both attach to the rear of the existing transmission, usually replacing the existing transmission’s rear housing. The resulting additional overall length will usually require shortening - or replacing - the original driveshaft. In some cases, minor chassis modifications may be needed.

•Having a complete installation done will run between $2500 and $3500 depending on which unit you go with and how hard the specific installation is. Be aware that a new driveshaft or other extensive modifications frequently are not included in a basic installation fee. A competent ‘shade-tree’ mechanic could install either.

•Having one installed will cost between $2500 and $3500 depending on which unit you buy and how hard the your installation turns out to be. Be aware that a new driveshaft or any other modifications frequently are not included in a basic installation fee that are quoted. A competent 'shade-tree' mechanic could install either.

•Both have their own oil supply and normally will not affect the factory transmission's operation. (However, if you have a late model transmission that's heavily computer managed, you should check whether it's fully compatible or if there are operating constraints.) .Both companies strongly recommend a specific oil to use in their unit.

•Both have electric shift controls. The GV box mounts on the dash (and has the option of using a foot-operated switch) and the USG switch mounts on the shift lever.


•The US Gear Dual Range unit was originally designed and initially built by Doug Nash Transmissions several years ago. Internally it's a very conventional-looking husky two speed gear box that uses an electric servo motor to shift between its two ranges. It stays in the range it's left in, even in reverse or with the engine off. Because of its basic gear box design, it can be configured as a conventional overdrive or as an underdrive. (Older models at least can be reversed back and forth - but two of the gears need to be replaced to do that. There's also the issue of putting new gears in with worn. Also keep in mind that US Gear reportedly doesn't support the Mile Marker and Doug Nash manufactured units - those with serial numbers below 20,000 - if you're thinking about buying a used unit.) Shifting requires selecting the new range via the switch, waiting a second or two for the servo motor to pre-load the shift fork, then backing off the throttle momentarily to allow the unit to actually change gears. So it operates and acts very much like a two speed axle. The driver's experience and technique determine the smoothness of the shift - badly managed shifts tend to be somewhat jerky and noisy (it is not hard to learn to shift the unit smoothly however). There have been reports of the older units running hot under extreme conditions, but I have an article which claims that's almost always the result of overfilling and/or using the wrong oil. Repairs can be made by any mechanic who's competent with manual transmissions.

•The Gear Vendors unit is originally from England where it was used on medium duty trucks among others. GV apparently adapts the basic unit to the various US transmissions. Internally it's built more like a two speed automatic transmission than anything else and it's very impressive looking unit inside and out. It too has been around for many years. It uses planetary gearing to provide an overdrive ratio. An electrically operated valve hydraulically engages the overdrive planetary gear. An internal hydraulic pump is driven off the unit's output shaft and thus it needs a minimum of 20 mph or more road speed before it can develop enough pressure to upshift . Below its minimum road speed - or when overdrive isn't selected - it's spring loaded to lock into direct drive. Because of its basic design it can't be configured as a true underdrive. Very smooth shifts - up or down - also require some throttle management during the shift - backing off slightly during up shifts and accelerating slightly during downshifts. Also GV recommends not letting the unit 'self-shift' out of overdrive when the road speed gets too low (i.e., via loss of hydraulic pressure). They also recommend not using compression braking in overdrive with a heavy load. By using a speed transducer in the speedometer drive that reads road speed, this auxiliary can be set to upshift automatically - usually about 50 mph. Repairs should be done by someone with specific training on the unit or by the factory.


Gear Vendors -

• shifts under power
• semi-automatic operation
• overdrive only

US Gear Dual Range -

•available as an overdrive or underdrive
•stays in selected range at any speed
•requires ‘power pause’ during shifting


Both companies tout their 'gear splitting' abilities as a way to get up to twice as many gears. That, in my opinion, requires a lot of manipulation, coordinating the auxiliary transmission's shifting along with that of the main transmission. For example if you are in high direct and want to go down to second over, you have to shift both the main transmission and the auxiliary unit at the same time. It can be done, but my wife has trouble with that process still.

Keep in mind that with either unit, running your main transmission in second (or first) gear for long periods puts additional stress on the transmission and also generates more heat.

Neither is truly automatic in operation and both need conscious driver management for a smooth shift.

Properly used, both will provide added grade climbing performance. Actual mileage increases from people I’ve talked to seem to be all over the spectrum from none to some. In my opinion, justifying installing one on the basis of fuel savings is suspect in view of the substantial initial expense. You might even lose mileage if you use that additional hill climbing ability too aggressively (grin).

If you primarily need more power for climbing hills and not a higher final gear ratio, the USG underdrive might be a good choice since you can put it in low range at the start of the climb and then use the main transmission's gears as needed. If you need a true overdrive and will be shifting a lot, the nod might go to the GV unit since its shifting is a somewhat more forgiving.


Jerry asks:

I have read the information provided (By Tony H) on the RVer's Corner bulleting board regarding aux transmissions. It was very informative and helped me a great deal. However, I have a few more questions I'm hoping someone will provide answers to. I realize this is quite allot of information to digest and respond to. But, I'd greatly appreciate any inputs an experienced driver could offer, Thanks in advance for all inputs.

The Gear Vender will provide (when in overdrive) a higher gear ratio between the OEM transmission normal gear ranges. So in Drive, as the vehicle accelerates and if the GV is in "auto" each gear is automatically split higher ?

GV in auto operation,if the Transmission is moved from D to D1, you actually are moving the transmission to a ratio 22% higher than the D1 gear ratio which may be more efficient (depending on the grade, RPM, vehicle speed,etc) than dropping all the way to D1 basic?

When in manual mode and the electrically controlled valve is actuated with the foot control switch above 20 MPH the GV shifts into overdrive providing a 22% higher gear ratio for whatever range the transmission is currently operating?

When driving in the D-over range(E3OD trans) and a grade is encountered, when peak RPM and vehicle speed begins to decay, manually shifting the transmission to the D1 range will provide a ratio 22% higher than D1?

When peak RPM and vehicle speed begin to decay in D1-over, a shift to D1 would be appropriate by cutting out the GV with the foot switch? If lower gearing was still required, shifting to D2 and then back to D2 over with the GV would be appropriate? if still lower gearing was necessary, the GV could be cut out providing D2.

Understanding, that the GV could be disengaged at any time during a grade assent providing unsplit ranges and perhaps more efficiency?

When descending a grade, with heavy load, the GV should be disengaged with the foot switch as not to encounter compression (engine) braking?

When operating in the GV "Auto" mode, pressing the foot switch does what?

Thanks again for any information and experiences shared regarding the GV transmission.

Tony answers:

Tony H

No problem with the questions Jerry. Maybe this will help:

It's best to think of the GV (or the US Gear for that matter) as a second transmission that operates entirely independently of the main unit. So when the overdrive function is engaged, it applies the overdrive ratio to whatever gear that the main transmission is in.

If you know your main transmission's internal gear ratios, dividing those ratios by the overdrive ratio will give you the effective new ratio in that gear with the overdrive engaged. By comparing these new ratios with the stock ratios in each range, you can get an idea whether 'splitting' between one of the main transmission's shift ranges will give you a effective 'new' ratio.

The AUTO feature on the GV uses a sensor intalled in the speedometer drive (not sure how its read on electronic speedometers). At about 50 mph the overdrive upshifts independent of what gear the main transmission is in. Or what the vehicle is doing (like accelerating, coasting or whatever.) If you downshift the main transmission, the overdrive will still be engaged.

Similarly, the unit will shift out of overdrive if the speed drops too low to maintain lockup pressure. At that point, the next upshift will not take place until 50 mph is reached.

The floor button is used to downshift out of overdrive.

This is how my GV works on a TurboHydramatic 400 - others might be slightly different.

Frankly, I find the auto feature bothersome and so always shift mine manually. For one, it sometimes upshifts when I would rather it didn't. And two, to get a smooth upshift, you need to feather the throttle during the shift - and it usually cathes me unawares leavng me sometimes with a solid shift clunk when I'm towing. So now I have the only the engage and dis-engage buttons (no foot button) mounted on my seat console and do all my shifts (up and down) manually. Just my preference though.

Thanks for the compliment about the article. Free to e-mail me off line if I haven't made things clear.


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