How It Works: 2WD vs. AWD vs. 4WD

The vast majority of passenger vehicles on the road today use front-wheel drive (FWD), where the engine’s power is routed to the front wheels. Front-wheel-drive designs are cheaper to manufacture and more space-efficient than rear-drive systems because the engine, transmission, and axles are located close together in the engine compartment.

From an interior packaging standpoint, front-wheel drive precludes the need for a space-robbing transmission and driveshaft hump running down the middle of the cabin floor. Plus, FWD has the added advantage of better traction while climbing hills and driving on slippery surfaces because the engine’s weight is poised over the front wheels.

All-wheel drive is often added to FWD-based vehicles and power is sent to the rear wheels only as needed. FWD can also compromise a vehicle’s sporty performance, but it strikes a widely desirable balance for everyday driving and usability.

Rear-wheel drive (RWD) is commonly found on full-sized pickups and truck-based SUVs, as well as sports cars and high-performance and luxury sedans. For trucks, RWD allows the use of bulky, heavy-duty axles and suspension components that are designed to handle large loads.

On a performance car, rear-wheel drive improves handling by helping to balance the car’s weight more evenly front to rear—the heavy engine up front is offset by the weight of the rear differential. And because the front wheels don’t have to do double duty—both driving and steering—designers can optimize the front and rear suspensions for their respective roles.

However, RWD provides less traction on slippery roads. These days, most high-end cars offer all-wheel drive either as standard equipment or as an option. All truck-based (body-on-frame) pickups and SUVs are offered with optional four-wheel drive.

As the name implies, all-wheel drive (AWD) can feed power to each corner. Depending on the design of the system, AWD can provide maximum forward traction during acceleration. It is especially helpful in sloppy road conditions and when driving over moderate off-road terrain. It can help get you going and keep you moving through mud, sand, and other loose surfaces. Most AWD systems deliver power primarily to one set of wheels, front or rear. When slippage is detected at one axle, power is diverted to the other axle, in hopes of finding more traction there. 

Sophisticated clutch systems and, more recently, the use of electric motors allow for precise control over the power sent to each wheel. High-performance vehicles with AWD often send power to each corner at all times. But they can alter the balance among the wheels depending on conditions or drive mode to change the car’s performance characteristics. 

Not all AWD systems are equal. Subaru’s AWD system always directs at least some of the engine’s power to the rear, and it can direct a larger amount aft if needed. Many other systems fitted to front-wheel-drive vehicles operate with 100 percent of the power normally going to the front wheels; the rear wheels then receive power only when the front wheels start slipping, or proactively based on the vehicle conditions or drive mode selection. This transfer of power is traditionally achieved with a mechanical driveshaft running the length of the car, but electric vehicles and some hybrids utilize individual motors at each axle with no physical connection between them. Some performance cars that are offered with rear-wheel or all-wheel send more power to the rear wheels by default.

AWD systems are especially helpful in rapidly changing conditions or when driving on a road with intermittent snow and ice. It is commonly used for car-based SUVs, as well as certain cars and minivans.

Although four-wheel drive (4WD) and AWD are designations that are often used interchangeably in advertising and sales literature, there is a difference. Generally, 4WD is optimized for severe off-road driving situations such as climbing over boulders, fording deep water, and tackling steep hills with loose, low-traction surfaces. Most 4WD systems use a heavy-duty transfer case with a high and a low gear range, the latter used to increase torque at the wheels for low-speed climbing. Some have differentials (which allow left and right wheels and front and rear axles to turn at different speeds) that can be locked for maximum traction.

There are three types of modern 4WD systems
• Full-time, which means power is always sent to all wheels
• Automatic, where the vehicle automatically switches between two- and four-wheel-drive modes as needed
• Part-time, which requires the driver to manually shift between two- and four-wheel drive. Vehicles with a part-time system shouldn’t be driven in 4WD mode on dry pavement, because doing so could risk damage to the vehicle’s drivetrain.

Aside from serious off-road enthusiasts, most drivers never come close to needing the capability that 4WD systems provide over and above AWD systems.

For rain and very light snow, 2WD will probably work fine, and for most vehicles, front-wheel drive is the preferred setup and is likely to cost less than an equivalent AWD model. (For performance cars, RWD is preferred, but AWD, if available, can increase traction.) AWD is fine for most normal snow conditions or for light-duty, off-pavement excursions on dirt roads or slippery surfaces. If you’ll be driving in severe snow or true off-road situations, or if you’re interested in pursuing off-roading as a hobby, you should opt for a vehicle with 4WD and lots of ground clearance. Keep in mind that both AWD and 4WD systems add considerable weight to a vehicle, compromising fuel economy.

One of the reasons many people buy a traditional sport-utility vehicle is for the extra security and traction of four-wheel drive. Many drivers don’t realize the limitations of AWD and 4WD, however. Although having power delivered to all four wheels increases straight-line traction, neither system improves braking, and most systems have little to no effect on cornering.

Drivers can be fooled into traveling too quickly in slippery conditions with an AWD or 4WD vehicle, only to discover they are going far too fast when trying to stop. Because the added traction of 4WD can allow a vehicle to accelerate more quickly in slippery conditions, drivers need to be more vigilant, not less. Slippery conditions demand extra caution, no matter what you drive.

In many cases, having good tires is more important than the drive wheels. Winter tires, for instance, actually do help you turn and stop on a snowy road—things that AWD doesn’t help with.