Perhaps A Wee Bit Of Overkill,But A Great Tow Vehicle
When it comes to towing, there is a growing number of four wheelers who feel you shouldn't ask a pickup to do a truck's job, and with ever-heavier trailers and more trail toys to haul, it's probably a good idea to pull it all with something that actually has the strength, weight, and stopping power to do it safely. And, in this case, then some.
One believer in larger tow vehicles is Soni Honegger of Chama, New Mexico, who after breaking a few NV4500 trannies in his trusty Dodge Ram with its Banks-enhanced Cummins, decided to go with a stouter setup-strong enough to forever end any worries about breakage. Most would be content with a regular used and dirt-cheap class-8 Freightliner, but Soni had much bigger plans-big enough to get an Army surplus '80-vintage AM General 8x6, aka an M-920. Homely (to put it mildly) in stock form, these truck tractors were a Medium Equipment Transporter, an apparatus used to tow bulldozers on equally heavy trailers. While its GVWR is a relatively low 75,000 pounds, the 920's 99,730-pound towing capacity is certainly adequate for recreational use, and noticeably better than that of a so-called Super Duty. Maybe a standard- or medium-duty truck isn't all that bad after all?
With a driven Rockwell steer axle capable of supporting 20,000 of the M-920's 31,270-pound curb weight, just planting the air-suspended lift axle would support the whole thing on only four of the load-range-L Michelins, with over 8,800 pounds to spare. With the rear Rockwell tandem setup having a 58,000-pound capacity, Soni doesn't really need the lift axle and simply uses it as a convenient place to carry two spares, which can be lowered to the ground by flipping a dash-mounted switch. Considering that each 22.5x13 steel wheel and 46.3-inch-tall tire weighs a combined 420 pounds, having them there is probably a really good idea. Factory Detroit Lockers (technically, No-Spin differentials) turn 2 1/2-inch axleshafts in the rear axles, while the front axle is slightly weaker, and has an open diff.
Getting this mass moving is the job of a Cummins Big Cam 1 with just 400 ponies, so it's really the torque (1,250 lb-ft at 1,350 rpm) of the straight-six diesel that does the deed. With a single-speed transfer case, the engine needs other gearing to succeed, and with a 16-speed tranny, it does. Original axle ratios were 6.17:1, but even with the taller tires, Soni could barely speed in school zones, so he swapped in three sets of 4.88s. Now, the lumbering giant is able to reach a blistering 82.3 mph while bouncing off the governor at 2,100 rpm, and that's with the top gear being a 0.83:1 overdrive. Even after the gear change, in First gear that max rpm nets only 3.2 mph, so a low-range is, well, not essential. That air-shifted 16-speed Caterpillar tranny also deserves mention-it's a very uncommon transmission, and one of the few automatics you can actually grind gears with. Also, it uses a fair amount of the contents of the seven air tanks on the M-920 to complete a single gear shift. Its centrifugal clutch is more ordinary, a setup also found in Citroen 2CVs, for example. (Which, by the way, is another vehicle that's quite capable in the dirt, but unlike an M-920, because of its very low weight and supple suspension.)
Another notable feature is that the semi-ancient Oshkosh transfer case is operated by a switch on the dash (which also activates the inter-axle power divider), so that not-so-great idea isn't all that new.