Like sideburns, bell-bottoms and hip-huggers, rear-wheel drive is making a comeback. Almost ubiquitous during the go-go '60s, the design fell out of favor for passenger cars by the '80s and '90s. But now, new entrants such as the Cadillac STS, Chrysler 300 and Dodge Magnum get their power from their rear wheels and give real meaning to the term "retro."

A car's drivetrain means a whole lot more than a mere fashion statement, however. Perhaps more than anything else except the engine, the powertrain layout gives a passenger car its over-the-road personality. It affects steering, handling, cornering, acceleration and other characteristics that you probably think of as "driveability." It also affects fuel economy and traction.

For all those reasons, drivetrain layout matters when you consider which vehicles to test-drive for your next car. Increasingly, carmakers offer a choice of powertrain configurations in the same model. As a smart shopper, you need to know what to expect from various layouts.

Two in the Front
Front-wheel drive remains by far the most popular design for passenger cars. In this layout, the engine sits sideways, at a right angle to your path of travel. It sends power through the transaxle (a transmission-differential combo) to two drive axles (sometimes called "half-shafts"), each driving a front wheel.

From a designer's perspective, this arrangement has a couple of big pluses. For one, the axles are located underneath the engine, which allows designers to flatten out the intrusive driveshaft hump running down the center of a rear-wheel-drive car. But most important, the transverse-mounted engine permits engineers to maximize interior space without increasing the car's exterior dimensions.

Space-saving packaging explains why front-wheel-drive cars became so popular after the late '70s, when consumers and the federal government demanded better gas mileage.

"Manufacturers pared down vehicle size for fuel economy reasons," says Jim MacPherson, contributing editor of AAA's online new car and truck buying guide. "The components aren't as spread out as they are in a rear-wheel-drive car."

From a driver's perspective, front-wheel-drive cars also tend to accelerate better in snow than their rear-wheel-drive counterparts. "With the engine right over the drive wheels, the added weight gives you added traction," notes Frank Niland of the Club's Traffic Safety Department.

But better winter traction represents the only driveability advantage of this arrangement. Because the same set of wheels not only powers the car but also determines its direction, steering becomes more difficult in a front-wheel-drive vehicle compared with its rear-wheel counterpart. Front-wheel-drive cars also show a greater tendency to understeer--that is, the front wheels tend to go in a straight line as you take a curve.

Then there's "torque steer"--a bugaboo unique to front-wheel-drive vehicles. The term describes a car's tendency to pull to one side during sudden, hard acceleration. For instance, you find yourself fighting to stay on course while passing or merging with freeway traffic.

You feel torque steer because of those two half-shafts in front. "It occurs when you have unequal power application on the axles or axles of unequal length but equal rigidity," explains MacPherson. "One axle twists more than the other, so the car pulls to that side." The more powerful the engine, the more noticeable and troublesome torque steer can become.

Two in the Back
In a conventional rear-wheel-drive car, the engine in front transmits its power to the two wheels in the back through a long tube (driveshaft) running down the centerline of the vehicle. Because one set of wheels moves the car and another set guides it, steering feels much easier in a rear-wheel drive vehicle. It's no coincidence that the world's benchmark luxury cars--the BMW 7-Series, Lexus LS, Jaguar XJ and Mercedes S-Class--have always used rear-wheel drive.

Steering ease (and no worries about torque steer, regardless of engine power) helps explain rear-wheel-drive's recent comeback in certain market segments.

"There's a renewed emphasis on performance now," notes MacPherson. "You can put only so much power on the same set of wheels that you also ask to steer the car. Carmakers concerned with luxury and performance are returning to rear-wheel drive for some models."

The downside? Rear-wheel drive requires a larger overall package to yield the same interior room as a front-wheel-drive model. Size translates to weight, so fuel economy suffers. And rear-wheel-drive vehicles don't behave as well on slippery surfaces as front-wheel-drive models do. But technological advances such as traction control help to compensate.

Four at the Corners
Nowadays, all-wheel drive has become the hottest powertrain layout in passenger cars--hotter even than rear-wheel drive. According to CSM Worldwide, the number of front-wheel-drive vehicles offering all-wheel drive will jump to more than 8 percent of the North American market by 2007, up from barely 3 percent two years ago. And that doesn't even count cars designed as all-wheel drive only.

The all-wheel-drive systems in passenger cars send power to all four wheels, all the time. Unlike the systems used in pickups and truck-based sport-utility vehicles, they don't require the driver to decide when four-wheel power is needed.

All-wheel drive's superior traction only partly explains its exploding popularity. "Because it sends power to all four wheels, all-wheel drive exhibits better balance," says MacPherson. "It's one way of handling the performance requirements, with engines becoming more and more powerful."

You can measure the disadvantages of all-wheel drive in dollars--not only in the purchase price, but especially at the gas pump. Generally, you can expect gas mileage to drop 4 to 8 percent when you go from a two-wheel-drive to the all-wheel-drive version of the same vehicle. "You also have to consider the added complexity and perhaps higher maintenance," cautions MacPherson.

If rear-wheel drive seems like the retro style in fashion, then all-wheel drive is fast becoming the new black. But, like your choice in clothing, a car's drivetrain layout works best when it fits your budget, expectations and personality.