illustrators: G.R. Whale
The number of four-wheel-drive vehicles grows every year for cars, trucks, and those crossovers in between. And as that crossover line blurs, so does the one that defines four-wheel drive. So many "modes" and "tracs" are on new trucks that even we have to climb underneath to sort them all out. A study of these various drive systems will help you decide what advantages one offers over another--if any at all--list who has what, and ponders if Newton really was on to something.
If you learn nothing else here, learn this: Four-wheel drive does not nullify the laws of gravity, physics, or common sense. Regardless of drive, only the tire patches connect you to the ground, high-centering or falling over notwithstanding. If you blister down a snow-covered grade with only tenuous grip, you'll learn the hard way that for every action there is a reaction, as you thud to a stop in a snow bank.
A typical AWD system has a transversely mounted manual gearbox with an integral front differential, and a ring-and-pinion set to deliver power to the rear driveshaft.
Do You Need It? Four-wheel drive will help in acceleration and to a lesser extent in steering, because the front wheels are pulling in the direction preferred. But there's nothing to suggest a 4WD will stop better, as this is generally a function of brakes, tires, and vehicle weight. Four-wheel drives often come with better tires for adverse conditions and, in some cases, with better brakes (either upgraded to four discs or with ABS standard), but 4WDs weigh more as well, with some of that weight added over the already heavier front end. On the same tires with no brake differences, the 2WD will usually stop better.
A 4WD will offer more suitable traction, sometimes better ground clearance, occasionally improved ride quality derived from the larger tire sidewall, may sit higher for a superior view, and might win a "mine is bigger" driveway battle. For some trucks (Wrangler, Land Rover, Hummer), 4WD may be the only way you can get them.
On the other hand, 4WD will cost more (in the neighborhood of $2500-$3500). It will also cost more to keep, use tires and gasoline faster, and the extra driveline parts may add to routine--and unexpected--service bills. It will weigh more--anywhere from 200 to over 500 lb--depending on the model, and it may have a little less power because of packaging restrictions, especially in those AWD vehicles based on a front-driver. Finally, a 4WD might cost more to insure, though this is becoming less of an issue.