Most vehicles use two-wheel drive (2WD), where engine power is sent to either the front or rear wheels only. Front-wheel drive is used in most cars, wagons, and minivans because it's space-efficient. It allows a smaller engine compartment, leaving more room inside for passengers and cargo. It's also better than rear-wheel drive in slippery conditions because there's more weight on the front drive wheels and the wheels pull rather than push the car along the road. This helps prevent the vehicle's rear end from sliding sideways in slippery conditions.
Rear-wheel drive places less demand on the front wheels, allowing them to be used primarily for steering. It's commonly used on pickups and traditional, truck-based SUVs that are designed to handle heavy-duty chores such as towing. But rear-wheel drive is also popular on sports cars and high-performance sedans because of its contribution to good handling.
Traction control, available on many two-wheel-drive vehicles, helps maximize traction at the drive wheels by preventing wheel spin. It's particularly useful starting off on a wet, snowy, or icy surface. If neither drive wheel has grip, however, traction control won't help. In wintry conditions, we've found that all- or four-wheel drive is better than traction-control alone for getting up a slippery slope.
All-wheel drive (AWD) feeds power to all four wheels. It provides maximum forward traction and is especially helpful in wintry conditions and when driving over moderate off-road terrain. AWD systems are especially helpful in rapidly changing conditions or when driving on a road with intermittent snow and ice. Its lightness and compactness makes AWD the system of choice for wagons, some minivans and pickups, and most car-based SUVs.
The limitations of all- and four-wheel drive often go unappreciated. Power to all four wheels helps if you're going in a straight line but does nothing to improve braking or cornering. Thus, such systems don't enable you to drive the same way or at the same speed as you would on a dry road.
Although four-wheel drive (4WD) and AWD are designations that are often used interchangeably in advertising and sales literature, the key difference is that 4WD incorporates low-range gearing, which, when selected, helps in more challenging off-road conditions, such as climbing over boulders, fording deeper water, or tackling steep off-pavement hills. The vast majority of 4WD-vehicle owners, however, never come close to needing this capability. 4WD systems are also more expensive, more complicated, and heavier, which compromises fuel economy.
Modern 4WD systems are either full-time, which means they can stay engaged all the time, or automatic, where the vehicle automatically switches between two- and four-wheel-drive mode, depending on the driving conditions. Many pickups and some truck-based SUVs, however, have only part-time 4WD systems. These require the driver to manually shift between two- and four-wheel drive, which limits the vehicle's ability to provide optimum traction when the road surface suddenly becomes slippery.
For rain and very light snow, 2WD will likely work fine. Front-wheel drive with traction control is the preferred setup. AWD would provide an additional margin of safety. AWD is also fine for most normal snow conditions or for traveling on dirt roads without high rocks, deep sand, or steep inclines. If you're going where you'll encounter more-severe conditions, you should opt for 4WD.
In addition, a vehicle with a part-time system can't be driven on dry pavement when in 4WD mode without running the risk of severe damage to the vehicle's drivetrain.
Keep in mind that both AWD and 4WD systems add considerable weight to a vehicle, compromising fuel economy. See our list of best off-road vehicles.