In an ideal situation, all four of your tires would wear out at the same time. However, if you have forgotten to rotate your tires, your vehicle is out of alignment or you have worn suspension components, you may end up with a situation where only two tires need replacing. For information on properly checking your tires' tread depth, check out, "What Honest Abe Doesn't Tell You About Minimum Tread Depths."
What should you do if two tires still have adequate tread depth remaining and only two need replaced? First, be sure to match tires. Ideally, all four tires should be the same brand and model. Just as true as with a poker game, four of a kind always beats two pair. After you've found the two matching tires to replace on your vehicle, you'll want to make sure they're installed on the rear axle. This is true for rear-wheel drive, front-wheel drive and all-wheel drive vehicles.
Conventional thinking may suggest that new tires be put on the front to help prevent hydroplaning and reduce wet stopping distances. However, the combination of new tires up front and worn tires on the rear can cause some very unpredictable handling characteristics in the wet or snow, which can lead to an oversteer condition that looks like this:
Remember, it is best to install new tires on the rear axle. Members of Tire Rack had the opportunity to experience this phenomenon at Michelin's Laurens Proving Grounds. Participants were allowed to drive around a large radius, wet curve in vehicles fitted with tires of different tread depths -- one vehicle with new tires on the rear and half-worn tires on the front and the other with new tires in the front and half-worn tires on the rear. To see learn more about our experience and placing new tires on the rear axle, read "Where to Install New Pairs of Tires?"