4WD systems have drawbacks. They're typically bigger and heavier (often adding more than 200 pounds) than AWD systems, and they usually create more friction. Part-time systems without a center differential also have no "auto" mode, and hence when the weather or road surface conditions degrade, the driver must remember to proactively engage 4WD, perhaps after coming to a complete stop. That's vastly less convenient than an AWD system.

Basically, 4WD is of very little use on-road, so if you never plan to venture well off the pavement and into the rough stuff, you'll likely be better off with AWD. There are a couple of obscure on-road 4WD benefit cases to be aware of, however: 4WD transfer cases almost always include a "neutral" position, which disconnects both axles from the powertrain. This makes it safe to flat-tow the vehicle with all four wheels on the ground, so if you're one of those road-train RV vacationers, take note. Low range could also be useful for towing a heavy boat out of the water up a steep, wet boat launch.

Note: Jeep offers a quasi-4WD system called Active Drive Low. This setup, standard on Compass and Renegade Trailhawk and optional on other variants of those models, simply involves a shorter (numerically higher) axle ratio (4.33:1 vs. 3.73:1), which means that the nine-speed automatic's 4.71:1 first gear provides a reasonable crawl ratio of 20.4:1. That's a decent ratio for light rock climbing, but because there's no low-range multiplication in the other gears, we don't consider it a full-fledged 4WD setup.

Trucks and SUVs that currently offer true 4WD systems include:

All-Wheel Drive (AWD)

All-wheel-drive systems started out primarily as a foul-weather countermeasure that sent most of the power to one axle most of the time (often the front) then fed power to the other axle whenever wheels at the primary axle began to slip. More recently, they've also been employed as a means of improving the driving dynamics of front-drive-based sport sedans and utilities. Systems bringing AWD to a rear-drive-based architecture with longitudinal powertrain arrangement typically use a transfer case mounted to the back of the transmission to split power and send some of it forward.

Electrification brings yet another category of AWD to market—one that fits an electric motor on one or both axles in a hybrid or fully electric vehicle. The Porsche 918 Spyder uses an e-axle in front; front-drive-based e-AWD vehicles like the Volvo XC60 and XC90 T8 PHEV and the Toyota RAV4 Hybrid put it at the rear. These systems are less suited to off-road use—especially the hybrids—because depleting the battery can limit the power available at the e-axle and hence the all-wheel tractive force available.

2015 Porsche 918 Spyder front three quarter in motion 031

Setting the electrified AWD systems aside for the moment, the front-drive-based AWD systems tend to be the lightest, most fuel-efficient setups available (typically weighing well under 200 pounds and reducing EPA combined economy by 1-3 mpg). The most efficient new systems can disconnect the propeller shaft that runs between the two axles, reconnecting it in a matter of milliseconds when traction needs arise. During steady-state cruising this greatly reduces the amount of energy lost to friction and rotational inertia.

By their nature, AWD systems have the capability built in to be driven full time on dry pavement, and because so many of them have a torque-on-demand feature, some pretty ingenious power takeoff setups have been employed through the years.

Viscous Coupling

This simplest of center differential systems consists of a set of closely spaced discs—some attached to the front axle, others to the rear—surrounded by a special fluid that essentially solidifies under the shear force of those moving discs to lock the them together when there's a certain speed differential between the axles. Some manufacturers build in a bit of speed differential by making the front and rear axle ratios slightly different (the front-drive-based Land Rover Freelander worked like this) so that a bit of torque was always routed to the rear. Subaru Symmetric All-Wheel Drive has long used this setup with its manual transmissions.

BorgWarner/Haldex

When slippage occurs in this system, a ring turns, causing balls to travel up little ramps to create a clamping force that locks a wet multiplate clutch back that transfers torque to the secondary axle. Today's systems also include elaborate electronic controls. First used in the 1998 Audi TT, Haldex units are now widely used across the VW group (mostly in transverse-engine vehicles but also in the Lamborghini Aventador LP 700-4 and Bugatti Chiron), in most Volvos (except T8 models), and in the Ford Fusion and earlier Buick LaCrosse and Regal.

2019 Lamborghini Aventador SVJ side in motion 1

Torsen

A portmanteau of torque-sensing, these differentials use internal gears (helical or planetary) to apportion torque according to a predetermined ratio in such a way as to send the most torque to the wheel/axle with the best grip. Audi Quattro models with longitudinal drivetrains use Torsen center differentials, as do those that enable the auto-AWD functions on the 4WD Lexus GX, Toyota Sequoia, and Nissan Frontier Pro 4X.

Electromagnetic Control Device (EMCD)

Another way to achieve a variable front/rear torque split is simply to use an electromagnetic ram to vary the pressure applied to a wet multiplate clutch pack, like the one in a Haldex coupling. EMCDs are also used as limited-slip devices to control the torque split of an open planetary center or spider-gear front/rear differential.

On-Demand Rear-Axle Couplings

Acura's Super Handling All-Wheel Drive was first to market with this idea, which replaces the rear differential with a simple ring-and-pinion set to turn the power 90 degrees toward the left and right wheels, then uses electromagnetically controlled planetary gear sets to send power to either or both wheels on demand. The neat trick with the SH-AWD system is that the planetary gears can overspeed the wheel on the outside of a turn to provide quite noticeable torque vectoring. More recently, GKN's Twinster system uses simple EMCDs to power each rear wheel. In its late, great Ford Focus RS application, by giving it a slightly different rear ring/pinion ratio, fully locking either clutch overspeeds that wheel in much the same way, enabling the RS' "Drift mode." Note that most GKN Twinster axle fitments do not employ this overspeeding concept, in vehicles like the Lincoln Continental and MKZ, Cadillac XT5, current Buick LaCrosse and Envision, Range Rover Evoque, and Land Rover Discovery Sport. Obviously, simply adding a clutch at the front of the prop shaft gets you the fuel-saving benefits of isolating the prop shaft and differential gears when no rear torque is needed.

All of these systems make their own torque distribution claims, but the extreme numbers quoted are always based on some ideal set of circumstances that may be impossible to replicate in the real world. Certainly any claims of a front-drive-based system sending 100 percent of torque to the rear axle are bunk except possibly when the front axle is off the ground. Look for buttons to lock the center differential, as this guarantees a 50/50 front/rear split, which will usually enhance traction in the worst of conditions.