Contemplating an upgrade to your truck’s drivetrain, tires, or suspension? Then you’ll want to check out this fascinating and informative read by truck camper owner and off-road guru, Jeff Reynolds. Jeff outlines the steps, the “nuts and bolts,” and lessons he learned upgrading his truck camper rig.
I recently completed the final piece of my extreme truck camper (XTC) drivetrain build. Sean at River City Differentials in Cordova, California changed the differential gear ratios from 3.54:1 to 4.10:1. I’m finally a happy camper. Why the change? It’s really the feel you get with the different loads you put on the truck and whether you need taller (lower numerically), or shorter (higher numerically) differential gears, or, can you live with what you have. Let’s review the process.
I purchased a new 2001.5 Dodge 2500, 4WD, Cummins HO, that has an NV5600, 6 speed manual transmission with a 27 percent overdrive. This is important to know in developing a re-gearing strategy. In stock form the truck came with 265/75R16 tires (31.5-inch diameter) on 7.5 inch wide inside/bead to bead cast aluminum wheels, 3.54:1 pigs, 7,125 pounds with the driver and a full tank of fuel. With no load the stock drivetrain was the bomb. There was plenty of acceleration once the turbo spooled up. It was my daily drive for two years in Los Angeles traffic. I put 50K miles on the rig during those two years and enjoyed 18-19 mpg.
Subsequently, we started to flat tow our rock crawling Jeep Scrambler to off-road venues around the western United States. With an empty truck bed flat towing the 4,500-pound, up armored Jeep, the tail sometimes wagged the dog. The woe while flat towing was safely stopping the train. On the way to Moab, we almost lost it on Highway 70 on some of those 8 percent downgrades when the tail wanted to wag the dog. Then we bought a used Lance Camper which added another 2,600 pounds (loaded) to the rear axle of the truck and the tail wagging stopped. We then purchased a car trailer in order to more safely carry the Jeep, towed by the truck, loaded with a camper in the bed. Now we’re up to 16.5K pounds of rolling thunder. It was then that I finally broke down and bought a little commuter car and parked the truck.
BUILDING FOR BEEF
A couple of years ago, we set out to beef up the front axle. I half-heartedly began to look at the specs for a used Dana 70 front axle. It is built to the 7.5-inch backspacing with a 10 bolt wheel pattern of 4 and 5 series DRW trucks. This was not going to work. There was another option which was the Dana Super 60 which had bigger bearings, a heavier case, and larger ring gear and pinion. These being rather rare, I started to survey available components and finally concocted my derivative—a faux Dana 70—of which only the original housing and the 9.75 inch ring gear and 29 spline pinion remain at Dana 60 specification. The 2000 to 2002 Dodge Dana 60 front axle had 31 spline outer stub shafts and 32 spline, 3-piece axle shafts. Tyler at Geared Up Drive Train installed a Spyn Tec Dodge Dana 60 Hub Conversion Kit, inner spindle bearing kit, new Dana 70 spindles, 35 spline outer stub shafts, Mile Marker Dana 70 Lockout hubs, as well as factory ABS sensors and studs, Mosier 35 spline front Dana 70 long and short axle shafts, Generation 4 tie rod, drag link, TRE’s, Pitman arm and stabilizer, and new ball joints. He also installed a front 35 spine Eaton Detroit True Trac torque biasing, gear driven limited slip, deleting the central axle disconnect (CAD) and front unit bearings. Using ball joints may have saved Chrysler Corp. a few pennies, but added a sack of woe to the front end of their trucks. The best version of the Dana 60 front axle is the Ford version with 35 spline axles, a high pinion and actual king pins, not ball joints.
During the build, and knowing I was going to appropriate a taller 35-inch tire, I installed Top Gun Customz longer control arms to the front suspension. They are the 6-inch-lift version, but installed on a 3-inch lift, the result of which locates the front axle forward 1.5 inch. This delivered the clearance for the scrub radius on a 2nd generation Dodge at the rear of the front fender opening that I needed.
Along the way I put Mickey Thompson 16 x 12 inch wide cast aluminum wheels on the rear axle. The first set of tires were Nitto Mud Grapplers in a 33×14.50R16 size. They wore like iron and were singularly the noisiest tire I’ve ever used. I could not wait for them to wear out, but they lingered on for four long years before the replacements arrived. Subsequently, I found some of the last of the Mickey Thompson 375/65R16 (33×15.50R16) super singles. These were the ticket–quiet, AT tread, the widest and likely the best tire I’ve ever had on a 4WD vehicle. Deflated to 20 psi, they were absolutely the best sand tire ever for a heavy rig. The contact patch was about 15 inches long and 17 inches wide. At low pressure that translates to a lot of flotation.
The 12-inch-wide wheels had a 3,600 pound load rating, which was marginal but never materialized as a problem until the chipping. The tires were heavy and were E-rated at 3,750 pounds, well within the weight limits of Dana’s 11,000 pound rear axle. Chrysler Corp. rated the same Dana 80 axle at 7,500 pounds, for safety reasons, mostly to do with the stock tire loading. My problem was the cracking and chipping of the cast aluminum wheels. The reason they took such a beating is that the rear axle track was too wide for narrow jeep trails. I hit all the rocks that were sloughed off to the side by much narrower rigs. But the tires were tough.
ARCHAIC 16 INCH WHEELS AND SAND
There is a special reason for sticking with 16-inch wheels and tires when everyone else is following the bandwagon to 17-inch and 18-inch wheels. We use our truck camper on sand, beach sand, sand dunes, and sandy roads. The advantage is having the experience to know when to quickly deflate the tires and how far to deflate, and, of course, how to get those big meats back up to highway pressure when returning to terra firma. This technique was developed during 50 years of using 4WD’s on sand. Below is a pic of my first sand stick. The motivational device? A 1949 Willys Ute Wagon; Chevy 265 V-8; 5.38 gears; Studebaker 33 percent overdrive; Dana-27 and Dana-41 axles. This was the first of 14, four-wheel drives.
Sixteen inch wheels and tall tires give you the most sidewall for deflating on sand. When researching 17 inch tires I found that they, as a group, had less load carrying capacity. Eighteen inch had the highest load capacity of the 16-, 17-, and 18-inch tires. But they had a shorter sidewall, which was not as good for deflating. As good as they are on the highway, I did not consider 19.5 inch wheels and tires, since they cannot be successfully deflated for sand travel. Any “.5” wheel (16.5-, 19.5-, 20.5-inch) has a tapered bore cone-shaped rim, like a flower vase with no inner bead hump to help keep the tire on the wheel at low pressure.
Unsatisfied with aluminum wheels, it was time to investigate steel wheels. After looking around for high-capacity steelies, and coming up dry, I called Stockton Wheel and got in the queue to have four, 16 x 10 inch wide steel “Power Wagon”, 8 on 6.5 inch, lug centric wheels with two different back spacings installed. These had a stupid high load rating mostly because of the 1/2-inch solid steel plate center hubs. They are heavy (ugh), but should last into the next Ice Age. Yes, I know about unsprung weight and the negative effect it can have on acceleration, braking, and mpg, but I thought the pro of the extra capacity would off set the con of the unsprung weight. If I were to do a wheel swap again, I would go with 18 inch wheels and install the tallest tires that would fit in my fender wells to eliminate the disc brake fitment problem. Besides, I’m getting older so speed and acceleration are receding from my lexicon.
Knowing that most people who have wide truck wheels have a very shallow back spacing (3.5-inch to 4.5-inch) and how that application over time chews up the front wheel bearings and causes bump steer because of the side loading on the spindles, I opted for the front 10-inch wheels to have the stock Corp. backspacing of 6.25-inch and the rears to have a wider track via the 4.5-inch backspace. Alas, in the end the front 6.25-inch B.S. was not going to work. Why? The Stockton wheels use older heavy-duty DOT approved rims that have a neck down after the first 4 to 4.5-inch from the inside bead. Here is a comparo between a stock Dodge 7.5-inch-wide, inside bead to bead steel wheel on the left and the 10-inch-wide Stockton Wheel, inside bead to bead steelie on the right. Note the quick neck down on the right wheel (coming up from the floor mat) and the extended wider area on the left wheel which fits comfortably over the disc brakes, front or rear.
Now the woe. The front Stockton wheels with the 6.25-inch backspacing would not clear the disc brakes. It looks like they will be relegated to a wide layer in the Burgess Shale. I talked to my off-road fabricator, Brother John (JR on the Pirates of the Rubicon site), and he said you could grind down about an 1/8-inch of the brake parts to make them fit. He was in town and took a look to see if making the wider wheels fit the front axle was feasible. His pronouncement after crawling around under there was, “Not without a lot of grinding, and even then, iffy and probably not worth it.” I then started to see alternative front steel wheels and to that end dropped my spare tire down, which had a factory steel wheel. The wheel looked like it was H.D. enough to have a high load capacity, with its heavy stamped steel and welded hub with small cut outs. I could find no specs on this wheel except they were a rare commodity made in Brazil and used exclusively on Dodge 2500’s as the steel spare or the lowest trim level truck from 2000 to 2002. I settled on two, 7.5-inch wide, inside bead-to-bead, steel wheels with a 6.25 inch back spacing on the front axle. I chased down a pair from a Sacramento Scrap Yard, cleaned them up and painted them. Here is the front and rear view below. Note the big back spacing which gets the offset nicely over the wheel bearings.
I decided to go with a set of Cooper Discoverer LT315/75R16 AT3’s. They would fit these disparate rims and roll at the same speed in 4WD. These are E-rated at 3,860 pounds each; 12.4-inch section width; 34.49-inch in diameter; 9.8-inch tread width; weighing in at a heavy 63 pounds, per tire.
I drove the new 35 inch tires/wheels around for a week trying to get over how tall the final drive gearing now became with no load in the truck. One fact loomed large: the inability to use 6th gear. So, I searched the net to find out what optional gearing was available for this truck. It was then that I came upon Grimm Jeeper and their terrific gear ratio calculator. You simply select your transmission, put in two of the three numbers (speed, ratio, or diameter), and out come other parts of the gearing equation. After running the calculator my choice was between 4.10 gears used by Dodge or 4.30:1 gears used as an option in Ford F-450’s, but would still fit in my cases. The fact that my 6th gear is 27 percent overdrive and the engine pulls best between 1,700 and 2,000 rpm swung me over to the 4.10’s. When running the calculator, I noticed that the engine was ostensibly running right at peak torque at 1,600 rpm in 6th, at 55 mph with 3.54’s and 32 inch tires. Change the numbers: 1,607 rpm in 6th, at 55 mph with 4.10’s and 34.5-inch tires. Just a coincidence to have it that close. Sean at River City Differential told me on the phone that he had a slightly used set of Dana 60/Dana 80 RGP’s that he could install saving me a lot of cash on parts. The ring gear and pinion are always a married set. A used set needs no break in time. I motored down the hill at 80 mph, for the last time, and left the truck off for him to do his magic. Actually, he only had the front Dana 60 used parts and had to install a new Dana 80 RGP. Sean called and said my beloved Power Lok for the Dana 80 was now obsolete and no spare parts were available. The Power Lok would not fit on the thicker 4.10 carrier flange without a lot of machining anyway. He gave me a $350 credit for my old Power Lok, carriers, and RGP’s. All was not lost as he installed another Eaton Detroit True Trac in its stead. Actually I like it better as the tires do not chirp in tight corners and it is more transparent on the highway.
I’m now in the middle of a break in period where I’m not going fast; nor accelerating harshly; and driving only 20 to 30 minutes at a time, followed by a cool down period for the RGP. There is some metallurgical mystery happening if the gears get too hot or have too much torque applied during the first 500 miles.
So, I’ve come full circle from the beginning of this article. I said I’m liking the result with the 4.10’s but there is more there than meets the eye. The Gross Vehicle Weight with the camper on is 10,400 pounds. Does my engine have enough power to sustain a road speed? Other considerations: Tire diameter and unsprung weight. Heavier Wheel size and unsprung weight. Wind resistance at speed. 27 percent overdrive top gear. The 4.10’s are stronger, on paper, than the 3.54’s, mostly due to the size of the pinion. On my year truck, the GCVWR was higher by just changing to 4.10’s. These are all parts of the build puzzle and have their hand in the result. With the camper on now, it feels just right and I can use the overdrive 6th gear again and don’t have to downshift as much on hills. The 245 HP and 505 TQ of the 24 valve Gen II HO Cummins, which seemed so powerful at the time, was built at the dawn of the Diesel Wars, and is now an anemic slug compared to the latest generations of diesel power on the market. The clutch should last longer with lower gears as the friction patch per shift is less especially when starting out. There is one more item that comes into play here: age. I’m no longer a hot-rodder or a crazy, death-defying rock crawler, so our adventures with the truck camper contain more leisure than ever. I set out to improve the traction, clearance, and over-built-ness, and the ability to camp, stealth camp, and travel during any season in any weather under any road (or not) condition, for months on end, and park in a regular parking place along the way to resupply.