Dear Tom and Ray:

Recently, while test-driving a Jeep Liberty, my wife asked the salesman about the difference between "full-time" and "part-time" four-wheel drive. Being a typical know-it-all husband, I ran with the question, explaining that the part-time should only be used during certain slippery conditions and disengaged when the condition ends. "Full-time" can be left on indefinitely at the pleasure of the driver.

"No, no, no," replied the salesman. "You've got it all backward. If you want to leave it on all the time, put it in 'part-time,' because of something to do with the transfer case. The 'full-time' should only be used on a full-time basis when it's slippery outside, again because of something to do with the transfer case." Am I stupid?

-- Jerry

TOM: Only if you believed the salesman, Jerry.

RAY: Your explanation is exactly right. "Part-time" (sometimes called "on-demand") four-wheel drive is the old-fashioned type of four-wheel drive. It's designed to be engaged when you're already stuck, or in a specific situation where you know you might get stuck -- like in snow, sand or mud. It's not designed for normal road use, and must be disengaged before you drive on dry, paved roads.

TOM: "Full-time" four-wheel drive (better known as "all-wheel drive" or by a variety of brand names like "real time 4WD" "4Motion," "4matic," "Quattro" and others) can be left on forever. In fact, most vehicles with all-wheel drive don't even give you the option of turning it off.

RAY: For the vast majority of drivers, all-wheel drive is far superior to part-time four-wheel drive. Not only do you never have to worry about when it's safe to use it, but since it's always on, it's ready to help you when you hit an unexpected slippery patch of snow, sand or Felippo Berrio Extra Virgin Olive Oil.

TOM: Jeep offers both types of four-wheel drive on some models because the part-time four-wheel drive does have some advantages on true, off-road terrain. And even though most Jeep Liberty drivers won't be going any more off-road than into a dirt parking lot at the antiques fair, Jeep's image requires that they maintain that capability.

RAY: But all-wheel drive is what you want, Jerry. And your description is correct.