Bobby drives a 1992 HUMMER from AM General.

MHis Hummer has a 4-seat half-door open-cab 1992 model, colored black with tan interior and highlights. It has the standard undersized Hummer brushguard, a 12,000 pound Warn electric winch, extensive rocker panel and underbody protection, auxiliary PIAA spot and fog lights, and a swingaway spare tire carrier that also mounts 5' Hi-Lift jack. Also, production staff replaced the stock Hummer axles, namely Zexel-Torsen torque biasing differentials based on the AMC 20, with ARB Air Lockers. To simplify maintenance and improve cabin space, the production staff hacked the interior cabin extensively. For tires producers ran Interco Swamper TSL SXs during the mud season, and Goodyear Wrangler GSA radials during drier seasons.

Several Hummers were used in production. Due to the high volume of scars, dents, and poorly repainted patches, a day on the set of Renegade could allow one of these fine vehicles to pass for a military surplus model. Personally, I think it looks fine that way.

Where To Get A Hummer | How It Drives | How Much It Costs | About Maintenance | Specifications | Bibliography

Where To Get A Hummer

The Hummer

If you're looking for a used military model, try checking the advertisements in Military Vehicles magazine, or try the publications of the Military Vehicle Preservation Association, Army Motors and Supply Line, reachable at 800-365-5798. (You might find a driveable or almost-driveable military surplus model here, or here.) Be warned, however, that the US DOT in its infinite wisdom has decreed HMMWVs inherently illegal to drive on public highways, due to nonstandard lighting, lack of proper emissions control equipment, and a failure to pass the side collision test. Furthermore, the US military scraps almost all of their surplus trucks, vending the crumpled and cut remains through the Defense Reutilization and Marketing Service. (If reassembling a HMMWV from scrap appeals to you, try the DRMS National Sales Office at 800-222-DRMS.) Here we have a fine example of our tax dollars hard at work: I know I certainly feel safer - don't you?

You can find good values in used civilian Hummers by calling around the dealerships, locating those dealerships through the listings on the Hummer Mailing List Home Page, or by phoning AM General at 1-800-REAL4WD. Lynch Hummer advertises a large body of pre-owned Hummers, as does Attila's Used Hummer Network. Due to the minute number of Hummers sold, there is no stable aftermarket price.

When shopping for a used Hummer, pay careful attention to the condition of the control arm ball joints and the steering linkage, all of which wear out rapidly. Also, if the truck has CTIS, check the quick-disconnect valves at each wheel. When released, these values should disconnect the wheel from the CTIS, leaving the tire inflated; if, instead, the tire deflates, then the o-ring in the valve has stripped, necessitating that you replace the valve.

(Please don't ask me where you can get an inexpensive used Hummer, or where to get a surplus military HMMWV. Everything I know about buying a Hummer appears here. If it doesn't appear here, I don't know it.)

How It Drives

Hummer Front Seats

On-highway, a Hummer drives pretty much like a 2-ton dually diesel pickup truck. Off-highway, however, a Hummer is extremely nimble, particularly for a vehicle of its ample dimensions and great weight. Its high clearance and steep approach and departure angles let it ignore many obstacles. Every important powertrain component lies above and behind the frame, and often behind additional protective shields, minimizing the truck's vulnerability to underbody trauma. The torque biasing (think of them as limited slip) diffs that it carries both fore and aft let it operate much of the time with the center diff unlocked, allowing it to turn in a very small radius. And while Cummins fans may sneer at the small General Motors diesel, this plant has the torque where it's needed, down in the low rpm band.

Nevertheless, we're talking about a 3.5-ton, 15' x 7.5' truck. Imagine a full-sized dually: now extend the extra width of the dual rear wheels to the entire length of the truck, and you have an idea of a Hummer's dimensions. It has problems negotiating narrow forest trails, tight switchbacks, hill climbs on very loose soil, and downtown enclosed parking garages.

On-Highway Performance

The Renegade Hummer boasts a consistent fuel economy rating of 11.5mpg, dropping down to 10-10.5mpg if I push the speed above 55-60mph. On the highway, it has a top speed of 71mph, or 70 if I remove the top, which increases the drag of the spare wheel the truck carries in the back. The fuel tank's 25 gallon capacity yields a working range of 250 miles per tank. To understand these numbers, think of the performance of a jeep with 5.24 gearing, then scale it to a 2-ton truck.

Hummers are noisy trucks. The combination of aggressive mud tires, a flat windscreen, a diesel engine that practically sits in the driver's lap, a fabric roof and half doors, the minimal sound insulation of the '92 cabin, and steel underbody protection plates that reflect drivetrain noise up into the cabin, together yield an earsplitting din on the highway.

Every year the new models run more quietly, as AM General adds more effective insulation. For some reason this disappoints me.

Automatic transmission and the masculine self-image

Many people are surprised to see the automatic transmission in a Hummer.

All Hummers, both military and civilian, use automatic transmissions. The old ones have TH400s; newer ('94 et seq) models have 4L80Es.

Modern automatic transmissions are just as stout as manual transmissions. The advantage of a manual transmission lies not in its imagined ruggedness, but in the degree of control it gives the driver over the gearing used, plus the obscenely low "granny gear" first that many manuals have. The former isn't an issue unless you're either a sports car driver or else a long-haul commercial trucker playing gear-splitting games on hills to eke out a little more fuel mileage (and, I imagine, keep yourself from going bug crazy from boredom). The latter is great for rock-crawling off-highway drivers and for vehicles just starting to move while towing extremely heavy loads.

Remember: a HMMWV was designed as a practical tool for the military. It's designed to be easy to drive, not to make its driver feel more butch (the latter being the job of AM General's marketing machine, plus the natural attraction of the Y chromosome to quasi-military toys). An automatic relieves the driver of a parameter to manage in a hostile environment. It also allows the vehicle to be driven more easily by a wounded soldier, though I don't think that was a design requirement. Too, many military recruits enter the armed forces having never learned to drive a vehicle with a manual transmission.

Practically, the automatic transmission lets the driver trick the HMMWV's stock torque-biasing axle diffs into locking by modulating the brake against the throttle. This wouldn't be possible if the driver had to keep a foot on the clutch pedal during the process. The truck would need some sort of hand brake control; even then, the driver would be juggling three controls instead of two.

ARB Air Lockers versus Zexel-Torsen differentials

Stock drivetrain differential geometry:

The Geometry of Bobby's Hummer:

A stock Hummer operates most of the time with its center differential unlocked, trusting to the automatic torque biasing of its axle diffs to keep its tires from spinning, seeing it through any obstacles that appear. Even on many trails it can leave its center diff unlocked. When the trail becomes so rough that the wheels lose purchase, the operator manually locks the center diff, and if necessary modulates the brake against the throttle to trick the axle diffs into locking. This gives the Hummer a degree of mobility surprising in a truck of its dimensions, and a casual "go-anywhere" capability that is very comforting always to have on hand.

In exchange for this loss of casual mobility, the Hummer now handles extreme terrain more capably. The brake-throttle modulation trick of a stock Hummer is very fatiguing when used for extended periods, sacrifices a good deal of power to fighting its own brakes, exacerbates brake wear, and when sloppily used can even shear the truck's halfshafts (imagine the angular momentum of a 150 pound wheel, spinning freely in midair as the truck loses purchase: what do you think would happen should the driver suddenly hit the brake?).

To summarize, replacing the stock Hummer axle diffs with ARB Air Lockers gives the Hummer greater mobility at the price of much greater vulnerability when unprepared.

Life With CTIS


How Much It Costs

Yes! Hummers are expensive; moreover, they are expensive to operate, though perhaps not for the reasons you might expect.

High Initial Price

Compared to standard passenger rides, Hummers are expensive trucks. While I've seen used units for $25,000, a new Hummer starts at over $60,000 for a stripped bare model, graduating up to over $90,000 for a loaded wagon model. For some reason, the models displayed at car shows are always the most expensive configurations, further burdened with absurd leather seats, power toothbrush holders, heated cup holders, and $12,000 automobile stereos. You can save a lot of money by instead selecting the two-seat utility model with a full-length external cargo bed and eschewing all the foofy luxe. Throw a topper on the cargo bed, and you're set.

Unfortunately, the purchase price haunts you long after you've signed the check. Initial price dictates insurance premiums, and in the form of depreciated value will moderate licensing fees.

Insurance quotes vary per company and the history and demography of the insured driver. Nevertheless, all other factors being equal, the high sale price of a Hummer translates to high insurance premiums. (If your insurance company gives you any nonsense about not being able to insure Hummers, find a different company. In the eyes of your insurance company, your Hummer is nothing more than a light truck or SUV, albeit a moderately expensive one.)

High Gross Vehicle Weight Rating

In Washington State, truck licensing prices seem to be pegged to the truck's GVWR as well as its depreciated value. The axles of a standard Hummer can carry 10,300 pounds at limit. As a result, producers paid almost $900 a year per Hummer for the privilege of licensing the Hummers.

High Maintenance Requirements

Any vehicle that ventures off highway will require more maintenance than a pavement princess. The Hummer is no exception to this rule.

For maintenance purposes, you can think of a Hummer as a standard GM powertrain surrounded by expensive Hummer-specific components. Powertrain maintenance is cheap, since it uses standard parts, and since diesels require little maintenance beyond changing fluids and filters. Hummer-specific maintenance, however, is a thirsty vampire that will do its best to drink you dry.

For example, take the Hummer's tires. Renegade used to run 37x12.5R16.5 Goodyear Wrangler MTs all production season. (Try finding tire chains for that size off the shelf.) Like most good mud tires, Wrangler MTs have a brief road life, ranging from 25,000 to 30,000 miles. The standard dealership parts network wants between $550 and $650 per tire. If you go bargain hunting, you can find them at the relatively low price of $375 apiece, but you still need to have those tires mounted and balanced. Your corner tire shop is unlikely to have the tools or skills to do either. If you sport the military two piece wheels with rubber runflats, you're looking at a quoted rate of two hours of labor per tire to mount the tires on the wheel.

Fortunately, as the number of civilian Hummers in service grows, the maintenance story improves each year. Some economies of scale are beginning to come into play, with additional vendors of parts and accessories offering their wares, and standard part and service networks recognizing Hummers. For example, Discount Tire of Bellevue, WA now performs work on split-rim Hummer wheels, mounting and balancing tires.

About Maintenance

Yet Another Hummer

The greatest hassle in the standard gauntlet of Hummer maintenance is checking the oil in the geared hub assemblies, and working on the tires and wheels. The fill plugs in the hubs are awkwardly located next to the wheels, on the inboard side, in such a manner that makes them difficult to access and check. Two of these hubs require that you remove the wheel in order to check the hub.

The wheels are a special joy all by themselves: on Bobby's truck, they consist of a two-part steel assembly bolted together with a proprietary pentagonal nut, holding a hard rubber runflat and a 37 inch mud tire. These wheels weigh 155 pounds apiece, which makes changing them a delight. Replacing the tire on the wheel requires special tools to disassemble the wheel and insert a runflat into the new tire before reassembling the wheel around the tire and runflat. More recent models of Hummer use a lighter, single-piece wheel, perhaps in concession to civilian drivers that don't have their own motor pool maintenance battalions.

Interior Modifications

An open-cab vehicle gets a lot of weather in its cabin. Taking the truck back to its ultra-luxurious military roots, producers modified its interior a great deal, both to withstand occasional rainfall and to increase available cabin space. Having a 1992 model gave them a head start in the minimalist race, since the earlier models of Hummer were so much less plush than more recent models.

To clean the Hummer, the crew had to point its nose downhill, remove accessories, cover the radios, open the floor drains, then turn a hose and scrub brush loose on the interior.


This section describes the intimate anatomy of the Renegade Hummer, being a 1992 civilian Hummer with a fair amount of extra rubbish bolted onto it. Other models will differ in many ways.

The original military M998 Cargo-Troop Carrier was a much a lighter vehicle, with a rated GVWR of only 7700 pounds. As the United States military has pushed the HMMWV into more roles, AM General has enhanced the drivetrain and suspension to create the M1097, the M998A1, the M998A2, and further variants.

The civilian Hummer represents a divergent path of evolution. Starting in 1992 from the military M1097 Heavy Hummer variant with 12V electrics, padded seats, insulation, and a civilian light and dashboard package, it has picked up many of the subsequent military enhancements while mutating ever further down the civilian convenience and comfort axis.

Weights and Measures

The Hummer carries a large, heavy winch (a 12V Warn Mil-12000) slung under its front bumper, and sports extensive armor plating down the rocker panels on either side and beneath the drivetrain linkage. These skew the figures for both the weight and approach angles.

Length: 185.3 inches
Height: 72 inches
Width: 86.5 inches without mirrors
Weight: approximately 7000 pounds
GVWR: 10,300 pounds
Approach angle: 47 degrees (the winch gets in the way)
Departure angle: 37.5 degrees
Ground clearance: 16 inches
Wheelbase: 130 inches
Track width: 71.6 inches
Fuel capacity: 25 gallons


The engine is a 6.2L (379ci) Detroit Diesel Allison V-8, the old General Motors diesel of many a GMC Suburban or Jimmy. It's rated at net 150hp (gross 165hp) at 3600rpm, with 250 ft-lbs of torque at 2000rpm. The bore & stroke is 3.98x3.82in, (10.1x9.7cm) and the compression ratio 21.5:1.

The alternator is a 124A Delco-Remy 10479847, charging two 12V Delco 78-770 batteries connected in parallel.

The starter is a 12V Prestolite MMO, rated at 6hp.


The transmission is a GM 3L80 automatic, also known as the Hydramatic TH400. It's rated to handle a maximum input torque of 451 ft-lbs, well over anything that the Hummer's power plant can muster. A TH400 offers three forward gears and one reverse, with no overdrive.

The transfer case is a New Process Gear Model 242 HD AMG, with an intercooler. This is a full-time four-wheel-drive transfer case with two speeds and a manually locking differential. It differs from the NP242 used in the Jeep Cherokee and Dodge Durango in its spline count and shaft diameter, and has a maximum output torque of 2340 ft-lbs instead of the 1486 ft-lbs of the original.

AM General derived the original Hummer axles from the venerable AMC 20 axle. Their continuous torque rating was 128 ft-lbs. The stock differential uses a Zexel Torsen torque biasing design. The production Hummer, however, runs ARB Air Lockers instead of the Torsens.

Geared hub units lie at the end of each halfshaft. This portal axle geometry provides several advantages. It allows the truck to run lighter drivetrain components and brakes to reduce weight, and keeps the axle diff housings high out of harm's way, since they need not lie in a straight line with the center of the wheel. Since the diff housing does not move, each side of the suspension can flex independently. Finally, a fixed diff eliminates a primary cause of Air Locker failure - having the air line snag or pop off the axle housing.

Gearing Ratios

Transmission: 1-2.48:1; 2-1.48:1; 3-1:1 R-2.08:1
Torque converter: 1.96:1
Transfer case: L-2.72:1; H-1:1
Axle: 2.73:1
Hubs: 1.92:1

Multiply all of these together, and you get a net crawl ratio of 69.30:1. Taken by themselves, the axle and hub yield 5.24:1, which explains some of the Hummer's poor on-highway performance. (Later models of civilian Hummer use a transmission with an overdrive gear to improve this.)

Steering, brakes, wheels, and tires

The steering is by Saginaw, with a variable ratio of 13/16:1.

The brakes are by Kelsey Hayes: hydraulic actuated, four wheel inboard mounted power disc brakes with a dual reservoir master cylinder. Each rotor is 10.5 inches (26.7 cm) in diameter. The parking brake manually activates the rear service brakes.

The wheels are 12-bolt, 2-part split iron assemblies, with beadlocks and a hard rubber runflat. From November through May I run 38x12.50 R16.5 LT Interco Swamper TSL SX bias-ply mud tires, while from May through November I run 37x12.50 R16.5 LT Goodyear Wrangler GSA radials. These wheel and tire assemblies weigh approximately 155 pounds apiece.

Central Tire Inflation System

A 12VDC, 1/3 HP air compressor by Thomas lies at the heart of the Hummer's OEM central tire inflation system (CTIS). [more TBW]


In this Hummer page, I've limited myself to describing the Renegade Hummer. For more information about other civilan Hummers, military HMMWVs, or off-highway driving in general, consult the following books and web sites.


Few books about the Hummer exist. I've provided links to order some of them. (If you know of any other references, please, tell me about them!)

AM General's HUMMER service manuals
You can go straight to the source with AM General's service manuals. Mail AM General Corporation, Service Parts Logistics Operations, 408 South Byrkit Avenue, Post Office Box 728, Mishawaka, IN 46544-0728 USA. Or telephone 800-638-8303, their customer service hotline. For Canadian or other callers outside the United States who cannot use toll-free U.S. telephone numbers, this is 219-284-2956.
HMMWV Technical Manual, Unit Maintenance
Aka Technical Manual, Unit Maintenance, Truck, Utility, Cargo/Troop Carrier, 1-1/4 ton, 4X4, M998 (2320-01-107-7155). Approved by the U.S. Army for public release, this is technical manual TM 9-2320-280-20, parts 1 through 3. The U.S. Air Force calls this TO 36A12-1A-2092-1, while the U.S. Marine Corps calls it 2320-20/7A. Pick your service and order away through a U. S. Government Bookstore, courtesy of the GPO.
HMMWV Operator's manual
Army TM 9-2320-280-10 (USAF TO 36 A 12-1 A-2091-1; USMC TM 2320-10/6A).
HMMWV Direct support and general support maintenance
Army TM 9-2320-280-34 (USAF TO 36 A 12-1 A-2092-3).
Armored Cav
For what it's worth, Tom Clancy includes a single page on the HMMWV.

Hummer Web Sites

Hummer Application Web Sites

A ghost town since I deadheaded two years of stale links. I'll repopulate this someday.

Touring in the American West

You CAN go home again.

Last modified: 8 April 2000

Copyright 1996-2000 Torgo. All rights reserved.