OFF-ROAD's HD Dodge tow/chase truck was in need of a gearing change due to an increase in tire size. Having the 6-speed manual transmission has enabled us to put off this upgrade for a while due to its extremely low first gear, but we were starting to miss our granny gear off road. The problem was that it's not every shop out there that has gears for the newer AAM axles found under 2003-to-current Dodge Ram 2500s and 3500s. Gears for these newer axles have proven to be tricky to get aftermarket replacements for, but we found a shop that has any gear option we could want: JT's Parts & Accessories has AAM axle gears from 3.42:1 to 4.88:1 ratios. The solution was to change out our 3.73 gears for some 4.10s we got from JT's. This may seem like a (numerically) low jump to make since 4.10s come standard in the automatic trucks, but 3.73s are what you get with the manual tranny Combined with 37-inch tires, the 4.10 gears would bring our effective gearing to about the same as from the factory.
Another issue we were having with the truck was that the rear factory limit slip differential had clearly been worn into an open differential. The torque from the modified diesel engine would easily spin the inside rear tire when accelerating and turning at low speeds. We didn't want a selectable locker because that would give us the options of being fully locked, or completely open - neither of which we wanted to drive with on the road. What we wanted is a limited slip differential, but one that wouldn't wear out.
Luckily there is another option for us - an option with helical gears instead of clutch packs. Eaton's heavy duty Truetrac limited slip is now being produced for the giant 11.5-inch AAM rear Dodge axle. The Truetrac uses gears instead of clutches to minimize wheel spin. Using a gear design eliminates the wear issues associated with clutches found in conventional limited slip diffs. This means that it will remain consistently effective over its entire life and the axle fluid will not be contaminated by the shedding clutch material. Once we had obtained the gears and master install sets from JT's Parts & Accessories and picked up our Eaton differential, we headed down to our local 4 Wheel Parts shop to have the installation completed. Unless you're experienced with gears installations, we'd highly recommend taking a newer truck like this to a shop like 4 Wheel Parts that has done countless gear swaps.
 The new 4.10 gears being installed...  The new 4.10 gears being installed are about 11 percent larger than the original 3.73 gears. This means if the motor used to spin at 1500 RPM at 65 mph it will now spin at about 1665 RPM. This is just enough to give us a bit more usable pulling power without sacrificing fuel mileage at cruising speeds. |  The Eaton Truetrac for our...  The Eaton Truetrac for our AAM 11.5 rear axle is one serious hunk of metal. This is good since it will be subjected to loads of torque and abuse in our truck. A quality limited slip differential is a great upgrade for any vehicle that is going to see a wide mix of on and off-road driving conditions. |  JT's Parts & Accessories sent...  JT's Parts & Accessories sent us these master install/overhaul kits to go with the new gears. These kits include new bearings, crush washers, shims, axle seals and paint for checking the gear mating pattern. Parts like the axle seals are not mandatory to replace but while you have the axle apart it only makes sense to perform this preventative maintenance. |
 Steve, a tech at the 4 Wheel...  Steve, a tech at the 4 Wheel Parts store in Westminster, Colorado, made quick work of tearing down the rear end. Since the replacement gears were not a drastic change he kept track of the original installation set up (mainly the shims to set the pinion depth) as a starting point for installing the new parts. This tactic paid off and it took him only about two hours to complete the rear install. |  A torch is a very useful tool...  A torch is a very useful tool while doing a differential swap. Here, Steve is heating up the tone ring that is responsible for the speedometer reading. A bit of heat and this ring expands enough to simply fall off the original differential housing. Then, if you are quick enough, you can place the heated ring onto the new differential and it will slip right on. Once it cools the tone ring will be solidly mounted on the new carrier. |  The mating pattern of the...  The mating pattern of the new gears is very important and can greatly impact the longevity of the parts. This yellow paint is included in the master install set that JT's Parts & Accessories sent us and is used to check the mating pattern. After applying the paint you rotate the gears a few times and the matting pattern becomes visible. This pattern is just about perfect, being located in the center of the face of the teeth. |
 Once the proper gear setup...  Once the proper gear setup is determined the old pinion bearing and race are replaced with the new ones. Here, an air hammer and press adapter are being used to remove the original bearing race. The old parts are used for setup purposes so that the new parts are not damaged in case you need to disassemble everything to add or remove shims to adjust the pinion depth. |  You'll need to first remove...  You'll need to first remove the brakes, hubs, and axles before dropping out the front differential. It is more involved than the rear differential, but with proper tools and knowledge it comes apart fairly quickly. The hardest part was getting the hubs out of the steering knuckles. The right-sized hammer (a big one) persuaded them out of place. |  The original front differential...  The original front differential is a completely open design that does not offer any wheelspin-inhibiting devices. In the middle of the carrier you can see the spider gears that enable the two axles to rotate at different speeds. A front selectable locking differential would be nice, but for the amount of time we spend at low speeds crawling (almost never) the cost couldn't be justified. |
 Setting the preload on the...  Setting the preload on the pinion bearings is an important step when installing gears. The preload is measured by how much torque is required to rotate the pinion. An inch-pound torque wrench, like the one shown here, is needed for this step. |  Refilling the axles with a...  Refilling the axles with a high quality synthetic axle fluid was the last step in the installation. Royal Purple makes some of the best fluid available and is good insurance for our new parts. Now we just have to take it easy on the gears for a few hundred miles to allow the new parts to break in and then it is back to normal use. |  How Did It Drive? Finally...  How Did It Drive?
Finally changing the axle gear ratio on our truck had a really positive effect on the drivability - actually more than we expected. Once again we can start in second gear when the truck is unloaded. And the engine's rpm in overdrive are at 1650 when going 65 mph (which is exactly at the torque peak).Eaton's Truetrac limited slip differential is one of those upgrades you will probably forget about after installing. Testing in various conditions has shown it to be smooth, quite, and very good at limiting wheel spin. We like that it does not need to be activated, like a selectable locker, and is always there to help with forward progress either on the road or off. |