When is it time to buy new tires? Should the wheel be balanced when the new tires are mounted? If so, should they be balanced statically, or dynamically? Do you need a new valve stem, or is the old one okay?

These questions often trouble motorists. Knowing the answers can help you make wiser decisions when it's time to buy new tires for your car. And also help you avoid getting the feeling that you're being ripped off for service that isn't needed or, even worse, not having things done that should be done because you think you're saving money (when in the long run just the opposite is the case).

It's a wise car owner that checks the tire pressure in each tire once a week. Keeping the tires inflated to the recommended pressure (found in owner's manual, shop manual, or on a placard on the door post, glove compartment, or engine compartment of the car) and driving sensibly (no hard cornering and no drag-racing starts) will assure maximum tire life.

Tires should also be rotated as recommended in the owner's manual. Rotation means putting the front tires on the back and the back ones on the front at 6,000 to 8,000 mile intervals to help equalize tire wear. If your owner's manual does not cover rotation of tires, ask the service manager at a local dealership for guidance.

Radial tires must never be "Xed" - that is, the left front wheel can go on the left rear, and the left rear on the left front, but neither left wheeel can be put on either the front or rear of the right side of the car, and vice versa.

How do you know when tires are due for replacement?

Tires have wear bars, flat spots that appear across the entire width of the tire tread after they have worn enough tread off to require replacement. When the wear bars are visible, tires should be replace.

Driving "bald" tires increases the chances of a blowout and drastically reduces control on wet roads.

Okay. Let's say it's time for a new set of tires. What tire service should you have performed along with the purchase and mounting of the new tires?

NEW VALVE STEMS - Some car owners feel they're getting ripped off by paying for a new valve stem when the old one works perfectly well, so they don't have a fresh valve stem installed.

Valve stems are pretty cheap, though, and it makes a lot of sense to have a new one installed along with the new tire. Considering that you've paid good money for a new tire, it's kind of silly to try to save a couple of bucks by using the old valve stem, which may fail and cause you to ruin a perfectly good tire. A new valve stem is a lot cheaper than a new tire.

WHEEL BALANCING - Have each wheel balanced, either statically or dynamically. Static balancing is cheaper. When done properly, it will suffice for most tire-wheel assemblies. There are times, however, when static balancing will not do the job and dynamic balancing is called for.

Never run new tires without having them at least statically balance. Unbalanced wheels create shake and vibration, and they can wear out new tires quickly. CAPTION: Picture, no caption