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Off-Road Diesel Truck Showdown: GMC Sierra 1500 AT4 vs. Ram 1500 Rebel

Motor Trend logo Motor Trend 11/4/2019 Motor Trend Staff

a car parked on the side of a mountain: 2020-GMC-Sierra-1500-AT4-20.jpg
Used to be, four-wheel drive and some cushy shocks turned a pickup truck into a factory built off-road truck, but those days are long gone. Z71s and FX4s are just checkboxes now. If you want to drive off the lot and onto the dirt, you can get anything from a Baja-ready Raptor with long-travel suspension to a mountain-conquering Power Wagon with a factory winch. What you might really want, though, is something in between, like a new 2020 GMC Sierra 1500 AT4 or a 2020 Ram 1500 Rebel.

Research the GMC Sierra 1500 on MSN Autos | Research the Ram 1500 on MSN Autos

Ram's been doing the factory off-roader thing for 15 years with the Heavy Duty Power Wagon, but a few years back it broke new ground with something less extreme. Called the Rebel and based on the light-duty truck, it was more aggressive than your run-of-the-mill 4x4 off-road package but not too extreme. For the new generation, the formula remains the same: black out the grille, add a manually locking rear differential, give it off-road shocks and beefy tires, give the optional air suspension a higher clearance setting, and protect it with underbody skidplates.

Competitors have seen the popularity of the Rebel and have jumped in, with the sharpest elbows coming from GMC with the new Sierra AT4. Copying from Ram's homework, the AT4 has chunky tires, a 2-inch lift, underbody skidplates, Rancho shocks, and an auto-locking rear differential. GMC finally giving the Rebel a competitor is reason enough for this off-road truck comparison, but both brands offer an extra enticement: diesel.

a red truck driving down a dirt road© Motor Trend Staff

Duramax vs. EcoDiesel

Although Ram has been selling its EcoDiesel V-6 turbodiesel longer than it's been selling the Rebel, you could never get both on the same truck. Now, with the next-generation truck and engine, you can. Not missing a beat, GMC has also made its new Duramax I-6 turbodiesel available on the AT4. Off-roaders have been pining for diesel engines for years thanks to their stout low-rpm torque, which makes it easier to climb obstacles. And just like that, we have two options instead of none.

Before you can off-road, though, you have to get out to the country. Whether that means lots of highway or lots of two-lane blacktop, you'll arrive with the same amount of fuel in the tank either way. The GMC's EPA highway rating isn't quite as good as the Ram's, but its better city rating makes up for it, and both trucks get the same 24 mpg combined rating.

The GMC diesel, though, feels like isn't as stressed. The Duramax responds instantly to the throttle, giving you effortless, linear power dispatched quickly and seamlessly by its 10-speed automatic transmission. The Ram has power, too, but it's lazy at low rpm so you're always putting a lot of boot to it to get moving. The Ram's eight-speed auto is just as smooth as the GMC's and cleverer, too, throwing downshifts under braking to help slow the truck down and keep you in the power in case you need to speed back up. Clever or not, the slightly heavier Ram is noticeably slower across the board in acceleration testing.

It's not just the EcoDiesel that's a little lethargic, either. The Ram's brake pedal has more squish and takes more travel to get a good response, whereas the GMC's responds immediately with a nice, firm pedal. That said, both trucks stop from 60 mph in the same distance; the GMC just feels more confident about it. Likewise, the GMC's steering responds more quickly and accurately than the Ram's, which is a bit slow and ponderous. Even still, the Ram did manage our double lane change emergency maneuver slightly quicker than the GMC, albeit with a lot more drama and effort.

When you finally get off the pavement, you're presented with starkly different approaches to off-roading. Although Ram's parent company has plenty of high-tech off-road features in stock, the Rebel keeps it old school with a two-speed transfer case, a manually locking rear differential, and, if you spec your truck like we did, air springs that can raise it up for extra clearance. Beyond that, it's up to you to figure out how to get down the trail without getting stuck. There's no off-road button here, and even though Ram has a surround-view camera system, it's not available on the Rebel.

a car driving down a dirt road© Motor Trend Staff

GMC takes the opposite tack, throwing all the bells and whistles at the AT4. An off-road mode changes throttle, transmission, and stability control calibration, and an auto locking rear diff activates as needed without driver input. The surround-view camera system has a mode that lets you view the nose camera only, which is great for going over crests where you can't see over the hood. The information screen in the instrument cluster has pages that show your angle of inclination and roll as well as which wheels are slipping at any given moment. If you have less seat time off-road, or you just like having as much information available to you as possible, the Sierra AT4's got you covered.

Whichever way you go, you'll get there. The GMC's factory-installed 2-inch lift gives it 10.9 inches of claimed ground clearance to Ram's 10.2 in its highest suspension setting, though the advantage is more than wiped out by the side steps that are part of most option packages. Whether it's sharp crests, off-set moguls, or just a rock or log in the path, the AT4's steps dragged constantly, whereas the Rebel never made contact. If you're serious about off-roading your truck, take the time to choose every option you want rather than picking from GMC's predesigned packages.

Once you're all crossed up on the trail, you'll find yourself a bit more comfortable in the Sierra AT4. It's not just the peace of mind those cameras and info panels can give you, but the actual ride quality from the Rancho shocks. When the Ram's air suspension is stretched out to its maximum height, it has less travel than the AT4's, and the air springs are as stiff as they can get. As a result, you feel every tire impact much more sharply in the Rebel, and you're thrown around a lot more in the cabin. Our Rebel's front suspension also made a lot more creaking noises off-road than the AT4's.

Both trucks were fitted with Goodyear Wrangler DuraTrac tires on 18-inch wheels, but the AT4 often felt like it was using them more effectively. In situations that didn't require a diff locker, the AT4 seemed to have more grip, though locking up the Rebel's rear diff instantly made up the difference. When it came to obstacles that did require a locker, we preferred the Rebel's manual control that kept it on full time. GMC claims its auto-locker reacts within milliseconds when it's needed, but our experience varied from milliseconds to full seconds. Auto-locker and side steps aside, the AT4 went everywhere the Rebel did.

It's another matter if you want to bring your toys with you. The AT4's 320 pounds of additional payload capacity as configured lets you bring a lot more stuff in the bed and cab, and it doesn't slow down nearly as much as the Rebel when loaded with 1,500 pounds of cargo. Whereas the Rebel can tow 710 pounds more, the gap to the quicker AT4 only grows longer when you put 7,500 pounds on the hitch. In our special test of uphill passing while towing, the AT4 pulled even further ahead of the Rebel, accelerating from 35 mph to 55 mph. Not only quicker, the AT4 feels smoother and more confident towing than the Rebel, which had a tendency to surge and developed a vertical oscillation in the rear over bumpy roads.

a red truck driving down the road© Motor Trend Staff

When you're loading the toys in the bed or the toy hauler on the hitch, Ram and GMC both have special tricks to help. The Rebel's air suspension can be lowered to drop under a hitch and pop back up, saving you some cranking on the trailer jack. Both offer fancy multi-function tailgates, and we found the Ram's barn door solution, when combined with a kick-down step under the corner of the bumper, more useful. The step is lower and available whether the tailgate is open or closed, and the barn door function lets you get closer to the cargo you need to unload from the bed so you're not leaning over. The GMC's six-way tailgate does a lot of neat things, but the step doesn't work if the tailgate is up or if you have a ball hitch installed (even without a trailer attached it'll smack into the hitch). The optional built-in Bluetooth stereo speakers are cool, though, and we appreciated the extra bed capacity and tie-down points, plus the indestructible carbon bed.

Once everything's loaded in the back, it's time to pile into the cab. This is where the AT4's big liability comes into inescapable focus. We've lamented it before, and we have to say it again: The mildly updated and mildly bloated interior design of this new generation of Sierra (and Silverado) is a big letdown. The seats are uncomfortable, the materials are subpar across the board, there aren't enough USB ports for a modern family, and storage space is criminally lacking and underutilized. All that aside, the Sierra AT4 does win some points back with its Denali-spec interior, intuitive infotainment system, and litany of optional features. We particularly appreciate its laundry list of towing apps and special cameras, the full-color, high-resolution head-up display, the adaptive cruise control, the full suite of active and passive safety systems, and the surround-view camera system. We also like how quiet the AT4 is in the cab and how smoothly it rides. GM says its engineers tweaked every facet of the Sierra's (and Silverado's) suspension for 2020, and it shows in the ride and handling.

The Ram, as we've also often said, is way ahead of its class in terms of interior design and execution. The Rebel dresses it up with special color inserts and trim, but that's immaterial to the general excellence. The seats are comfortable, there's storage and power for everything, the materials can be downright fancy for a truck, and there's no truck (or many SUVs or cars, for that matter) that can touch that optional 12.0-inch infotainment screen. The truck isn't without its faults, though. Significantly more tire noise makes its way into the Rebel's cab, along with a booming noise at freeway speeds. In comparison to the AT4, we found the air suspension to be more brittle over rotted roads, which didn't use to be the case.

a car parked on the side of a mountain: 2020 GMC Sierra 1500 AT4 vs 2020 Ram 1500 Rebel comparison test 1© Motor Trend Staff 2020 GMC Sierra 1500 AT4 vs 2020 Ram 1500 Rebel comparison test 1

So Which Is the Better Off-Road Truck?

Can a fancy interior and tailgate enough cover for the Ram 1500 Rebel's shortcomings? For some buyers, yes, as rising Ram sales will attest. When it's time to put these trucks to work, however, the AT4 shines. It drives better, it tows better, it hauls better, and it off-roads just as well, it just does it a bit differently. An interior overhaul is probably well underway in the halls of GM's design department, and when it gets here, Ram will have real trouble on its hands, because GMC already has the fundamentals right. Work, play, and everything in between, the 2020 GMC Sierra AT4 is the truck to have.

Research the GMC Sierra 1500 on MSN Autos | Research the Ram 1500 on MSN Autos

DRIVETRAIN LAYOUT

2020 GMC Sierra 1500 AT4 (CrewCab 4WD)

Front-engine, 4WD

2020 Ram 1500 Rebel 4x4 EcoDiesel (CrewCab)

Front-engine, 4WD
ENGINE TYPE Turbodiesel I-6, alum block/headTurbodiesel 90-deg V-6, iron block/alum heads
VALVETRAIN DOHC, 4 valves/cylDOHC, 4 valves/cyl
DISPLACEMENT 182.6 cu in/2,993 cc182.3 cu in/2,988 cc
COMPRESSION RATIO 15.0:116.0:1
POWER (SAE NET) 277 hp @ 3,750 rpm260 hp @ 3,600 rpm
TORQUE (SAE NET) 460 lb-ft @ 1,500 rpm480 lb-ft @ 1,600 rpm
REDLINE 5,300 rpm4,500 rpm
WEIGHT TO POWER 20.4 lb/hp22.2 lb/hp
TRANSMISSION 10-speed automatic8-speed automatic
AXLE/FINAL-DRIVE/LOW RATIO 3.23:1/2.07:1/2.72:13.92:1/2.66:1/2.64:1
SUSPENSION, FRONT; REAR Control arms, coil springs, anti-roll bar; live axle, leaf springs, anti-roll barControl arms, air springs, anti-roll bar; live axle, air springs, anti-roll bar
STEERING RATIO 15.7:116.3:1
TURNS LOCK-TO-LOCK 3.03.3
BRAKES, F; R 13.5-in vented, disc; 14.1-in vented disc, ABS14.9-in vented disc; 14.8-in vented disc, ABS
WHEELS 8.5 x 18-in cast aluminum8.0 x 18-in cast aluminum
TIRES LT275/65R18 113/110Q M+S Goodyear Wrangler DuratracLT275/70R18 125/122R M+S Goodyear Wrangler Duratrac
DIMENSIONS
WHEELBASE 147.5 in144.6 in
TRACK, F/R 67.9/68.1 in68.5/68.1 in
LENGTH x WIDTH x HEIGHT 231.7 x 81.2 x 78.4 in232.9 x 82.1 x 75.8-79.6 in
TURNING CIRCLE 47.2 ft46.2 ft
CURB WEIGHT 5,661 lb5,769 lb
WEIGHT DIST, F/R 58/42%57/43%
SEATING CAPACITY 55
HEADROOM, F/R 43.0/40.1 in40.9/39.8 in
LEGROOM, F/R 44.5/43.4 in40.9/45.2 in
SHOULDER ROOM, F/R 66.0/65.2 in66.0/65.7 in
PICKUP BOX L x W x H 69.9 x 71.4 x 22.4 in67.4 x 66.4 x 21.4 in
PICKUP BOX CARGO VOLUME 62.9 cu ft53.9 cu ft
WIDTH BET WHEELHOUSES 50.6 in51.0 in
PAYLOAD CAPACITY 2,120 lb1,800 lb
TOWING CAPACITY 9,000 lb9,710 lb
TEST DATA
ACCELERATION TO MPH
0-30 2.3 sec2.3 sec
0-40 3.73.9
0-50 5.45.8
0-60 7.58.1
0-70 10.210.8
0-80 13.414.5
0-90 17.6
0-100
PASSING, 45-65 MPH 4.24.7
QUARTER MILE 15.7 sec @ 86.2 mph16.1 sec @ 83.7 mph
BRAKING, 60-0 MPH 130 ft129 ft
TOP-GEAR REVS @ 60 MPH 1,400 rpm1,700 rpm
CONSUMER INFO
BASE PRICE $60,310$52,735
PRICE AS TESTED $65,800$63,510
STABILITY/TRACTION CONTROL Yes/YesYes/Yes
AIRBAGS 6: Dual front, front side, f/r curtain6: Dual front, front side, f/r curtain
BASIC WARRANTY 3 yrs/36,000 miles3 yrs/36,000 miles
POWERTRAIN WARRANTY 5 yrs/100,000 miles5 yrs/100,000 miles
ROADSIDE ASSISTANCE 5 yrs/100,000 miles5 yrs/100,000 miles
FUEL CAPACITY 24.0 gal26.0 gal
REAL MPG, CITY/HWY/COMB N/A23.6/29.3/25.9 mpg
EPA CITY/HWY/COMB ECON 22/26/24 mpg21/29/24 mpg
ENERGY CONS, CITY/HWY 171/145 kW-hrs/100 miles180/130 kW-hrs/100 miles
CO2 EMISSIONS, COMB 0.94 lb/mile0.93 lb/mile
RECOMMENDED FUEL DieselDiesel
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