How much air should I put in my tires?

It depends on the vehicle application, the size of the tires, how much weight is on the tires, and whether fuel economy is more important to you than a smooth ride.

Listed in the owner's manual or on a decal in the glovebox or door jamb in every vehicle are the recommended inflation pressures from the vehicle manufacturer. For most passenger cars, minivans and minipickups, the recommendations range from 27 to 32 psi. For fullsize pickup trucks and sport utility vehicles, the recommended inflation pressures tend to be about 5 to 8 psi higher to reflect the larger tire sizes and greater weight of these vehicles.

It's important to note that the recommended inflation pressures may differ for the front and rear tires.

The manufacturer's recommendations are not necessarily the optimum inflation pressure for your tires, but are generally the best for all-round driving. Adding a couple of extra pounds of pressure will decrease the rolling resistance of the tires and make a slight improvement in fuel economy -- but it will also make the tires harder which in turn may cause a somewhat rougher or harsher ride.

If you're carrying a lot of extra cargo, car pooling, hauling a lot of stuff in the back of a pickup or towing a trailer, a few extra pounds of pressure would be recommended to offset the added weight. Add the extra pounds to the rear tires.

WARNING: Never exceed the maximum inflation pressure specified on the sidewall of the tire. This number is the maximum pressure the tire is designed to safely handle. Higher pressure increases the risk of tire damage (when hitting a bump) or tire failure.

Why Check?

All tires leak a little air over time, with some losing up to half a pound a month. If you're losing more air than this, you probably have a leak (possibly a rim leak or a porosity leak in an alloy wheel). For this reason, tire pressure should be checked at least once a month -- and certainly before taking a long trip or driving at sustained highway speeds.

Underinflated or overinflated tires can wear unevenly. Underinflation also increases tread wear dramatically.

WARNING: Underinflation may also increase the risk of tire failure or a blowout. When a tire with too little air in it (say 12 to 18 lbs.) is driven at highway speeds, the sidewalls are forced to flex excessively. This builds up a lot of heat in the tire which may cause it to fail.

How To Check

Recommended tire inflation pressures are always for COLD tires, which means you should check the tires in the morning before the vehicle has been driven. Driving heats up the tires and causes the air inside to expand. If you check the tires right after driving, therefore, the readings will be at least several pounds higher than normal.

Internal tire pressure will also vary with the ambient (outside) air temperature. Hot weather raises air pressure inside the tires, while cold weather lowers it. So air may have to be added or vented from the tire to compensate for seasonal variations as well.

Use an accurate tire gauge to check your tires. Don't rely on the built-in gauge on a gas station air hose or compressor (which tend to be very inaccurate). And never rely on your eyeballs alone to "judge" the amount of pressure in your tires. The sidewalls on radial tires typically bulge quite a bit even when the tire is properly inflated. If you keep adding air until the bulge is gone, the tire will be seriously overinflated. Likewise, don't wait until the tire is nearly flat to add air. It's nearly impossible to tell the difference between a tire that has 10 lbs. of air from one that has 20 lbs. of air. Use a gauge to check the tires regularly, and add or vent air as needed to keep the pressure within a couple of pounds of the amount recommended by the vehicle manufacturer.

NOTE: Tire inflation pressure should be more or less equal side-to-side. A difference of more than a couple of pounds may be enough to cause a noticeable steering or brake pull.

My mechanic says my suspension is not adjustable. Does that mean the wheels can't be aligned?

No, it simply means the vehicle manufacturer was too cheap to design adjustable suspension components when it engineered your car. When a car maker designs a car, engineers and accountants scrutinize each and every component to figure out how they can reduce manufacturing and assembly costs. If a few cents can be saved by leaving out an adjustable camber bolt, caster shim or whatever, they'll do it. They may have great faith in their own ability to build a vehicle that never needs to be aligned or fixed, but we all know from experience that such notions are untrue. So even though a suspension is nonadjustable and shouldn't require any corrections, that doesn't necessarily make it so. Even brand new vehicles can roll off the assembly line with wheels that don't meet their own alignment criteria.

Fortunately, the aftermarket has come up with ways to correct the "mistakes" of the vehicle manufacturers. If the car maker doesn't include provisions for adjusting the suspension, it creates an opportunity for some aftermarket part's supplier to come up with means of making such adjustments possible. These include offset bushings, shims, wedges and other alignment aids. So even though your suspension may have few if any adjustments for things like camber, caster and rear toe (front toe is adjustable on all cars and trucks), there are probably aftermarket alignment aids that allow at least some limited corrections to be made on nonadjustable suspensions.

What are all-season, asymmetrical and directional tires?

Essentially an all-season tire is a tire with a slightly aggressive tread pattern that provides good year-round wet and dry traction characteristics. It tends to be a bit harsher and louder than ordinary tires, but not as harsh or loud as a snow tire.

Some all-season as well as performance tires have a "directional" tread pattern. Directional tires have a "one-way" tread pattern that are optimized for the direction the tires rotate on the car. They must therefore be mounted on either the left or right side. Little arrows or triangles on the sidewall indicate which way the tire is supposed to turn. The tread blocks and grooves are angled to optimize handling. They also do a good job of channeling water out from under the tire on wet surfaces to reduce hydroplaning and improve wet traction. Directional tires can be rotated front-to-rear but cannot be rotated side-to-side.

Another variation in tread design you'll see is "asymmetrical" tires. Tires with an asymmetrical design mix tread patterns or put more rubber on one side of the tread than the other in an effort to make one tire out of two different tread patterns. Some combine a slick-like smooth tread on half the tire (to improve traction) with a block pattern on the other half (for directional stability).

Why are almost all tires today radial ply tires?

Because radial ply tires are better than bias ply tires.

The belts are layers of woven reinforcing fiber inside the tire under the tread. The belts give the tire strength, keep the tread from squirming and help resist punctures. The belts may be fiberglass, steel, nylon, rayon or aramid fiber.

In bias ply tires, which were common before radials were invented, the innermost plies crisscross like an "X" as they run from bead to bead (side to side), while the outermost plies run lengthwise around the circumference of the tire. This makes a relatively firm tread, but it also increases rolling resistance. And anything that increases rolling resistance increases fuel consumption.

The innermost plies in radial tires, by comparison, run sideways between the beads. This makes the tire more flexible, which reduces rolling resistance and improves fuel economy. Changing from bias ply to radial ply tires can improve fuel economy 10 to 15%, so that's the main reason why most tires today are radials.

The increased flexibility of radial tires also helps improve traction and cornering. Because the tire is more flexible, it is better able to maintain tread contact when cornering. This also allows lower aspect ratio tires (shorter sidewalls) which provide better handling performance.

When I'm driving down the highway, my car wants to pull to one side. How come?

A steady steering pull or "lead" to one side may have any of a number of causes. The most likely cause is wheel misalignment. This may be due to rear wheel toe or axle misalignment, front wheel camber misalignment, too much cross camber or caster alignment (more than a degree of difference side to side), or someone having "aligned" the front wheels without the steering wheel being properly centered beforehand. In any event, it will probably be necessary to have the alignment checked to diagnose and correct the problem -- unless one of the following is causing the pull:

An underinflated front tire on one side. Check tire pressures and make sure they are the same side-to-side (no more than a couple of pounds of difference).

Mismatched tires. Tires of different size, aspect ratio or even tread pattern on one side can create enough of a difference in rolling resistance to cause a pull.

A weak or sagging spring. Measure and compare ride height on both sides of your vehicle (measure at the fender openings). If one side is an inch or more lower than the other side, chances are you have a spring that needs to be shimmed or replaced.

A dragging brake. This can be caused by a frozen or sticking disc brake caliper that doesn't allow the pads to kick back out from the rotor or weak or broken return springs in a drum brake that don't pull the shoes back from the drum. Another possibility here might be a packing brake that isn't fully releasing on one side.

An uneven load. If you, your significant other or a passenger is causing your vehicle to lean to one side, it can cause the steering to lead in that direction. Don't laugh, a few hundred extra pounds can make a big difference in a small vehicle -- especially if the weight isn't evenly distributed side-to-side. If you can't do anything about the extra weight, it is often possible to compensate by having the wheels realigned with a "simulated" load positioned in the vehicle. Of course, then your vehicle may lead in the opposite direction if the extra weight is removed.

Excessive road crown. Roads are usually sloped (crowned) from the center towards the sides for drainage. If you spend a lot of time driving on highly crowned roads and find the constant lead to the outside shoulder annoying, you can have the wheels realigned to compensate for the excessive crown. Adding or subtracting camber from one wheel or the other to create a difference in the cross camber alignment of your front wheels can counteract this kind of problem.

I'm buying a new set of tires. Should I have them balanced?

Yes. Balancing helps to guarantee a smooth ride at highway speeds, and it helps to maximize tire life. An out-of-balance tire can be very annoying because it produces a shake that increases in intensity the faster you go. The up-and-down shaking of the wheel is hard on the suspension, not to mention your nerves, and also increases tread wear. An out-of-balance tire can develop a cupped wear pattern. So do yourself and your tires a favor and have them balanced.

Almost all service facilities and tire stores today use an off-car electronic spin balancer to balance the wheels. The tire and wheel are mounted on the balancer, then spun to find any heavy spots on the wheel. The balancer then indicates where weights (and how much weight) need to be placed to counterbalance the heavy spot.

Off-car spin balancers actually check two kinds of balance, "static" and "dynamic." Static imbalance causes a wheel to shake up and down as it spins, so static balance is achieved when both halves of the tire wheel assembly weigh exactly the same. Dynamic imbalance causes a tire and wheel to shake back and forth or sideways as it spins. Dynamic balance is achieved when the front and back sides of the wheel and tire weigh the same.

Do my wheels need to be realigned after the struts have been replaced?

On most vehicles they do. Here's why:

MacPherson struts are more than overgrown shock absorbers. They're an integral part of your vehicle's suspension. They replace the upper control arms and ball joints and serve as the steering pivots for the front wheels. When the strut assembly is unbolted and removed from the vehicle, the original alignment of the suspension is lost -- unless the position of the camber bolts and upper strut plate are first marked so they can be reinstalled in exactly the same position as before. But this only works if the same original strut is being put back into the car. If the strut is being replaced because it is leaking, damaged or worn out, the dimensions of the new strut will usually vary enough to cause a change in wheel alignment. So wheel alignment should at least be checked to see if adjustment is necessary (which it usually is).

On some import cars, the struts are "rebuildable." The housing has a removable nut that allows the old guts inside to be dumped out and a new cartridge installed. On these vehicles, it should not be necessary to realign the wheels after rebuilding the strut.