Most passenger vehicles on the road today use front-wheel drive (FWD), where the engine’s power is routed to the front wheels. In fact, all but a handful of SUVs are primarily front-wheel drive vehicles, with additional components that send some power to the rear wheels as the need arises. Front-wheel drive designs are cheaper to manufacture and more space-efficient than rear-drive systems. Plus, FWD has the added advantage of better traction while climbing hills because the engine’s weight is poised over the front wheels. From a packaging standpoint, front-drive also precludes the need for a space-robbing driveline hump running down the middle of the cabin floor.
Rear-wheel drive (RWD) is commonly found on pickups and old-school truck-based SUVs, along with sports cars and high-performance and luxury sedans. For trucks, RWD allows the use of bulky, heavy-duty components, and it provides better traction with a hefty load. On a performance car, rear-wheel-drive improves handling by balancing the car’s weight more evenly front to rear. And because the front wheels don’t have to do double duty—both driving and steering—designers can optimize the suspension for handling prowess. However, RWD provides less traction on slippery roads. These days, most high-end cars offer all-wheel drive either standard or as an option.
As the name implies, all-wheel drive (AWD) feeds power to each corner. Depending on the system (designs vary), 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.
Not all AWD systems are equal. Subaru’s AWD system always directs at least 20-percent 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 only receive power only when the front wheels start slipping.
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 have high and a low gear range, the latter used to increase low-speed climbing power. Some have differentials (which allow left and right wheels and front and rear axles to turn at different speeds) to be locked for maximum traction.
Modern 4WD systems are either full-time, which means they stay engaged; automatic, where the vehicle automatically switches between two- and four-wheel-drive mode; and part-time, which require the driver to manually shift between two- and four-wheel drive. Vehicles with a part-time system shouldn’t be driven on dry pavement when in 4WD mode, which 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 the rest of this great article and to determine which vehicle will suite your needs go to consumerreports.org