It wasn't so long ago that all-wheel drive was restricted to Jeeps, trucks, and the occasional esoteric supercar.

Now, according to data from Polk cited in The Detroit News, it's fitted to almost one third of the vehicles sold in the United States.

MORE: 2014 Jeep Cherokee: All The Details From NY Auto Show (VIDEO)

But AWD adds weight, complexity, and mechanical resistance to any vehicle that its two-wheel-drive counterpart doesn't have to cope with.

So as automakers add this popular feature to more and more vehicles, they must also work harder than ever to ensure that its impact on fuel efficiency is minimized.

all-wheel drive

all-wheel drive

One approach, used for years by Audi, Subaru, and other makers known for their all-wheel drive, is to design the drive system so a vehicle is driven primarily by its front wheels until sensors detect a loss of traction. Then the second set of driven wheels kicks in to provide power to all four wheels.

In the new 2014 Jeep Cherokee, now on sale, a new system designed by American Axle goes even further.

It adds a clutch between the transmission and the rear-drive components, so that no components of that drive system are being turned by the engine--which would sap power and could reduce gas mileage by 1 to 3 miles per gallon--until they're needed.

MORE: 2014 Jeep Cherokee, More Fuel-Efficient Crossover, Now En Route To Dealers

Then the clutch engages and the driveshaft, differential, and axle shafts are powered through the transmission to drive the second set of wheels.

2014 Jeep Cherokee

2014 Jeep Cherokee

The system is transparent to the driver, and the drive system's control unit will engage the AWD under more than 20 different circumstances, from rain to loss of traction on one or more wheels.

A 2014 Cherokee equipped with a 3.2-liter Pentastar V-6, nine-speed automatic, and base Active Drive I all-wheel drive system is EPA-rated at 22 mpg combined (19 mpg city, 27 mpg highway).

That's far better than the 17 mpg (15 mpg city, 21 mpg highway) achieved by the old body-on-frame Jeep Liberty that was replaced by the Cherokee.

All-wheel drive is now fitted to four out of five luxury models sold in the Northeast, as well as a majority of the compact crossovers sold in snowy states.

The 2014 Buick Regal compact sport sedan now offers all-wheel drive as an option for the first time; Buick spokesman Nick Richards says that 20 percent of Regals ordered now include AWD.


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