BY JEFF JOHNSTON
from Trailer Life Magazine (January, 1994)

Even with the increased availability of original-equipment overdrive (OD) transmissions in today's tow vehicles, there are still instances when having the option of choosing a gear between the factory selections would work better in a given towing situation. In such a case, an auxiliary gearbox may be just the solution needed to produce maximum towing efficiency.

The Dual Range auxiliary gearbox, manufactured by U.S. Gear Corporation, is a unit that can improve RV gearing versatility. The Dual Range is designed to fit behind many transmissions in today's tow vehicles and motorhomes, including rigs with Dodge, Ford and GM automatic and manual transmissions. Both OD and underdrive (UD) versions of the Dual Range gearbox are available. All Dual Range units have a 35,000-pound gross combination weight rating (gcwr), which is more than enough for most tow vehicles pulling trailers or motorhomes. The UD unit that we tested carries a [$2395] suggested retail price, not including installation. The Dual Range installation cost averages [$500, plus] shortening the driveline, and runs as high as approximately $600 for four-wheel drive vehicles.

Longtime readers may well recognize the Dual Range, which in the past was known as the Doug Nash overdrive and, later, as the Mile Marker auxiliary gearbox. Both of these earlier versions have had sort of checkered reputations. Thankfully, the new model has been totally revamped for increased performance and reliability, and features significant product improvements.

These upgrades were brought about by the fact that U.S. Gear is one of the largest American companies that specializes in the design and manufacturing of gear and drivetrain products. Company engineers looked at the Dual Range gearbox and found several things they could do to increase its reliability under towing conditions.

First, the gear pitch angle and size of the planetary and sun gears has been changed for closer tolerances and quieter operation. The number of teeth on the shift synchronizer engagement rings has been doubled, which increases strength, reduces backlash and helps provide smoother gear changes. All planetary gear-sets have been fitted with thrust-type roller bearings in place of the previously used thrust washers, and, finally, the gear-shifting electric motor is now a sealed unit for increased durability in the high-temperature, oil-bath gearbox environment. Also the design of the electrical circuitry has been improved for greater durability.
Our test gearbox was an UD model with a 1.25:1 gear ratio, which results in a 20 percent UD when engaged. We fitted the UD to a test truck with a 460-cid V-8 engine, a 3.55:1 axle ratio and a Ford E4OD automatic transmission with a .71:1 OD ratio. Use of the Dual Range gearbox netted the following approximate final-drive ratios, both in direct (DIR) and with UD engaged:

Features and Installation
The Dual Range is driver-controlled by a small switch unit that mounts on either the automatic or manual transmission shift lever, preferably in a spot where the driver's fingers can do the shifting. The switch includes a push-type gearbox engage/disengage button [and] indicator lights to show the unit's current operating status…

In most applications, the Dual Range replaces the transmission tail housing, engages the transmission output shaft via a splined coupler, and is a bolt-on installation. The stock transmission-frame mount is usually retained. It's necessary to shorten the driveline, a job that can be easily handled by a professional driveline shop or a machine shop.

Test Vehicles and Driving Route
The payload for the test truck was a 34-foot fifth-wheel trailer weighing 12,490 pounds wet. That figure exceeds the truck's 10,000-pound tow rating, but we chose the trailer for our short-term test because it represents a typical heavily laden tow vehicle that can use more torque, and we wanted to put the truck, and the UD gearbox, to a serious test. Under regular long-term use conditions, it's wise to stick to the manufacturer's recommended tow rating when matching a tow rig to a trailer.

Our test route covered some 735 miles of mostly flatland freeways, never exceeding 800 feet in elevation, with a section of two-lane state highways accented by 6, 7 and 8 percent grades.

On the Road: Freeway Cruising
One of the persistent complaints concerning early Dual Range models was the harsh, jarring shifts, both up and down. Part of the problem was due to the shift synchronizer ring design, which, as mentioned earlier, has been much improved in the current model, but it is also a question of consumer education. Very few users were adequately instructed about the proper shift procedure. U.S. Gear plans to change that with comprehensive instructions in each gearbox data packet. Dual Range dealers are also eager to make sure that their customers know how to change gears gracefully.

As demonstrated by a company representative, the Dual Range was shifted up and down several times with nary a whimper from the rest of the driveline or gearbox. The procedure is simple. To shift down, from DIR to UD in our case, the driver backs off the gas, pushes the shift button, then taps the gas to take the preload from the driveline, which allows the synchronizer to pop into -place. An upshift is just as painless. When cruising steadily, the driver hits the shift button while accelerating a little, then quickly taps off the gas for a moment to allow the gearbox to make its shift back to DIR drive.

Although it is fairly easy to do, it takes some practice to get it down as smoothly as the official U.S. Gear representative could do it. I found myself crashing the gears a bit at first, until I got the hang of the shifting sequence. After that, the process smoothed out considerably.

In DIR gear, the presence of the Dual Range is not apparent to the driver, with no additional drivetrain noise. Shifting into UD produces some gear whine, but not enough to be annoying.

Likewise, backlash through the driveline was greater than in the stock truck. The Dual Range has had much of its backlash eliminated as a result of its new components.

On the outbound leg of our test trip, we were fighting a 20-plus-mph head wind, which, conversely, gave us a tail wind advantage corning home-not unlike the conditions many trailerists encounter during vacation trips. This wind condition had a demonstrable effect on our freeway-driving results, due to the large frontal area on the Carriage trailer.

Driving into the wind, we were not able to use the Ford's OD gear with the Dual Range in DIR. The truck would shift back to third each time we tried to use OD. As soon as we shifted the Dual Range from DIR to UD, we were able to use the truck's OD gear when running at 65 mph. At 55 mph, we had to use -third gear because the truck would not hold speed in OD without shifting down.

On the way back, with a tail wind, we were able to cruise comfortably with the truck in OD and the Dual Range in UD at both 55 and 65 mph. We had throttle to spare for cresting most hills, and only the steepest grades called for a temporary shift back to third gear.

Even with the tail wind, the extra-tall axle ratio resulting from having the transmission in OD and the Dual Range in DIR did not let us maintain speed on anything but dead-level road.

Engine speeds with the transmission and Dual Range in OD/UD, respectively, were still slightly lower than with both boxes in DIR. In effect, use of the Dual Range UD gear has the same effect as having an intermediate gear between the truck's third and OD gears. (See the Gear-Ratio chart.)

Hill Climbing
We took on the 6, 7 and 8 percent grades with the transmission manually shifted to second gear and the Dual Range in both DIR and UD. Wind was not a factor during the hill-climb tests. The results are as shown in the Hill-Climb Performance chart.

In most cases, the Dual Range helped us crest the hills slightly faster due to the lower final-drive ratio when the truck was operated in second gear and UD. The only inconsistency in this chart is with the 6 percent grade results. At 3450 rpm, the engine is well above its 2200rpm torque peak, although it's still producing plenty of horsepower. At that speed, the engine sounds like it's overrevving. In short, the truck was moving fast enough that it ran out of usable engine rpm. That was not the case with the steeper hills, on which the engine was perking right along as it should.

These hill-climbing results are consistent with the recommended use data from U.S. Gear. It recommends that UD gear be used around town or in seriously hilly terrain where the lower-speed, low-gear pulling power is desirable; then the user might shift the Dual Range to DIR for most freeway cruising. As previously discussed in this article, our freeway drive showed that there are times when the UD gear would be handy on the open road, as well.

Acceleration
Our acceleration runs displayed minimal change between use of the Dual Range in DIR or UD. In DIR, the truck and trailer reached 60 mph in 30.9 seconds; when shifted into UD, that figure dropped incrementally to 29.5 seconds. Likewise, our 40- to 60-mph times were 17.7 and 15.6 seconds, respectively.

Fuel Economy
We averaged 7.3 mpg overall for the duration of our test trip. During the first part of the drive, with the tranny in OD and the Dual Range in DIR, we averaged 7.4 mpg. Covering the same route later on, with the tranny in DIR and Dual Range in UD, we recorded 7.8 mpg, a 0.4-mpg increase. That's close enough, given our fillup-to-fillup economy-measuring procedure, that we'd be hard-pressed to say the increase was due strictly to the Dual Range unit.

In Conclusion
As tested, the Dual Range auxiliary UD gearbox made some performance improvements in our truck's operation. Some of the steeper hills were taken at increased speeds. Of course, every driver and RV combination will produce different results and may come to conclusions not consistent with ours.
The Dual Range appears to be most useful when towing a trailer that seriously strains the tow rig's drivetrain. Similarly, a heavily laden truck with a moderate-size power plant, such as a small-block engine, could also benefit more from use of the Dual Range UD and the wide range of slightly lower final-drive ratios.
It appears that the U.S. Gear Dual Range has come a long way since its introduction many years ago. It's made great progress in the areas of durability and user-friendly operation. Properly matched to a tow vehicle or a motorhome that needs help in the powertrain department, it could be a real plus for improved RV performance.

U.S. Gear Corporation, 9420 Stony Island Avenue, Chicago, Illinois 60617
(888) 874-3275 or (773) 375-4900.