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The Ups and Downs of Inflation

Inflation may be an undesirable consequence of economic growth. But when it comes to tires inflation is always a good thing - proper inflation, that is.

The amount of air required to properly inflate a tire depends on the size and type of tire, the vehicle application (size and weight), vehicle loading (normal or extra loading), and driving conditions. A tire that is properly inflated will provide safe driving, maximum traction, good handling and optimum tire life.

Increasing a tire's inflation pressure beyond the recommended amount will reduce rolling resistance, thereby improving fuel economy. But the trade-off is a harsher ride and increased risk of tire damage when encountering bumps.

Excessive pressure may distort the tread to the point where it bulges like a donut, reducing contact with the road and increasing wear in the center of the tread. Under no circumstances should a tire ever be inflated beyond the maximum rating as indicated on the sidewall.

Cause of Most Tire Problems
By far, underinflation is a more common and serious problem. Reducing inflation pressure increases a tire's rolling resistance and hurts fuel economy. Plus, an underinflated tire flexes more, which leads to increased and uneven tread wear. As a rule of thumb, tire life decreases 10 percent for every 10 percent it is underinflated.

Underinflation also makes a tire run hot. Increased flexing of the sidewall increases the temperature of the tire, which in turn increases the risk of a tire failure and blowout.

A low tire can cause other problems as well. The amount of air in each tire affects weight distribution between the wheels. An underinflated tire doesn't carry its full share of the load. This, in turn, affects chassis loading, traction, steering, alignment and braking. It may also cause a noticeable steering pull when driving or braking.

An underinflated tire can also break traction more easily than one which is properly inflated, which can cause skidding during braking or hard cornering, or wheel spin when accelerating.

The Rights and Wrongs of Pressure

So how much air is the "right" amount to use? The simple answer is because every application is unique, it's best to always refer to the recommended inflation pressures specified by the vehicle manufacturer. These are generally listed in the owner's manual or on a decal in the glove box or door jamb.

For many passenger cars and light trucks, the recommended OE tire pressure may range from 28 up to 35 psi. Recommended pressures for front and rear may also vary, and higher pressures may be recommended for towing or hauling loads.

Keep in mind that recommended inflation pressure are for cold tires. This means tires that have not been driven on for several hours (ideally overnight). It also means tires that are at a normal outside temperature of about 70° F.

To accurately inflate a tire, you have to compensate for changes in temperature. For every 10° F change in ambient temperature, tire pressure will change a little more than half a pound.

A tire that contains 32 psi of air at 70° F will have a little over 35 psi at 100° F - even if the vehicle hasn't been driven. Take a quick drive down the freeway and heat up the tires even more, and the pressure may read 38 to 40 psi.

Likewise, when seasons change and temperatures drop, tires lose pressure. They haven't lost any air, but the air isn't exerting as much pressure as before. The same tire that held 32 psi at 70° F will have only about 28 psi when the thermometer hits 32° F. And when temperatures are in the subzero range, the loss in air pressure will be several pounds more.

Altitude will also affect tire pressure. For every 1,000 feet in elevation above sea level, atmospheric pressure decreases about a pound and a half. As a result, tire pressure goes up an equal amount.

Underinflation Robs Tire Life

As we said earlier, tire life decreases 10 percent for every 10 percent it is underinflated. A tire that normally requires 32 psi of air and is would normally go about 80,000 miles will lose about 8,000 miles of expected tread life at 29 psi (assuming the vehicle is properly aligned).

The same tire underinflated to 26 lbs. will lose about 16,000 miles of tread life. If underinflated to 22 lbs., tire life will decrease by at least 24,000 miles.

Tires and alloy rims typically leak a little air over time, and with radials it's hard to visually check if a tire is low or not. That's why inflation pressure should be checked once a month. Yet few motorists do, and consequently, over 50 percent of all vehicles have one or more tires that are under- or overinflated.

Inflation pressure should always be checked after mounting tires to make sure it is within recommendations. If a bead is slow to seat, it can be very easy to overinflate a tire. In trying to seat beads, you should never exceed 40 psi.

But it sometimes happens, so it's best to always check inflation pressure after the tire is mounted. And make sure you use an accurate pressure gauge. Gauges are often out of calibration, so either get them recalibrated, or help your customers by getting new gauges.

Larry Carley

NITROGEN GAS Tire fill scam

It's worth mentioning at this point that many tire shops will recommend a "Nitrogen" gas tire fill. This is a scam. Do not pay extra for a nitrogen tire fill. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.

Yes, nitrogen is technically a better gas to fill tires with than air, though it’s not really so much about nitrogen itself as it is not having water vapor in your tires. Is it worth it? For normal driving, no. The advantages, while real, are still really very miniscule and the premium that tire shops charge for it is not worth the price.

So, when a shop tries to sell you a Nitrogen fill they're just filling you with inert gas and their register with your hard earned money.

-Trunk Monkey
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