Story and Photography by Jim Marotta

 

 

The automotive tires that keep our vehicles rolling down the road are rated by the manufacturer to handle specific loads at a predetermined air pressure. By keeping the air pressure in the tire optimized, both tire performance and fuel economy are maximized.

So what makes it so difficult to keep tire pressure consistent? According to tire industry data, 85 percent of all tire air pressure losses are the result of slow leaks that occur over a period of hours, days, or months. Tires typically lose air pressure through natural leakage or permeation at a rate of about 1 psi per month. In addition, tire manufacturers say that seasonal climatic changes result in air pressure losses of 1 psi for every 10° F decrease in the ambient temperature.

Here in the Northeast part of the country, differences between summer and winter temperatures average about 46° F, resulting in a net loss or gain of nearly 5 psi in air pressure. This great variation is enough to dramatically affect handling, traction, and durability of the average tire if the pressure is not adjusted seasonally. Even temperature fluctuations during the day can make a dramatic difference, as variations between nighttime and daytime temperatures in this part of the country can average 20° F and result in pressure changes of over 2 psi. 

No matter where you are, temperature fluctuations, whether large (think: Arizona in the summer) or small, as in balmy Mumbai in winter, will affect your tire pressure, so careful monitoring is important.

During a February 2001 survey, crash investigators measured tire pressure on each vehicle coming into a gas station and compared the measured pressures to the vehicle's placard pressure. They found that about 36 percent of passenger cars and about 40 percent of light trucks had at least one tire that was at least 20 percent below the placard pressure.

According to US government statistics, 660 people die and 33,000 are injured every year due to tire pressure-related accidents. In addition, improper tire pressure costs an extra $3.7 billion in fuel annually and every year causes, 4.5 million tires to need replacement before their designed lifespan.

The Mechanical Gauge


In the beginning was the tire pressure gauge, a simple device that, once applied to the valve stem of a tire, would indicate the air pressure inside a tire. The most ubiquitous of its many variants was (and still is) the pen-style gauge. Here's how it works:

Inside the pen (tube) shaped body, a piston made of soft rubber is forced by air entering the gauge against a calibrated spring which runs the length of the tube. On the opposite end of the tube is a stop. The distance the piston travels is relative to the pressure in the tire.

Mechanical tire pressure gauge components.

While using this type of mechanical gauge is simple, keeping the tires at correct pressure is more difficult. First, there must be a convenient supply of dry, vapor-free compressed air to inflate the tire. This is not a problem for the technician in a shop but may present a problem for the some motorists. Second, the gauge in use must be accurate. Gauge accuracy may affect correct pressure by 3 psi or more. Last, most vehicle owners do not check tire pressure nearly enough. An American Automobile Association (AAA) poll suggests that 85 percent of motorists do not know how to check tire pressure.

The Tire Inflation Pressure Label

So by now you are convinced of the value checking your tire pressure regularly. The first step is to determine the correct tire size and air pressure for your vehicle. If you have a subscription to www.ChiltonDIY.com or www.ChiltonPRO.com, you can just check with Chilton for the proper specification. For vehicles built after 2002, a placard and label must be located on the driver's side B-pillar. If a vehicle does not have a B-pillar, then the placard and label must be placed on the edge of the driver's door. If the vehicle does not have a driver's side B-pillar and the driver's side door edge is too narrow or does not exist, the placard or placard and label must be affixed to the inward-facing surface of the vehicle next to the driver's seating position. Most people are not able to locate or do not even know about their vehicle's information placard and yet, this small sticker contains key information about the vehicle, including the correct tire pressure for the front and rear tires. In most vehicles, the owner's manual will direct you to the vehicle information placard's location.


The tire inflation pressure label contains gross vehicle weight, correct tire size, speed rating, and cold inflation pressure information.

Tire Pressure Monitoring Systems

Tire pressure monitoring systems (TPMS) report real time tire pressure information to the driver of the vehicle and are mandatory on all vehicles produced for sale in the US since September 2007. So if the vehicle is equipped with it, why not let the TPMS take care of the tire pressure? There is one very good reason: many of these systems only alert the driver to low tire pressure when the tires lose 25% of their pressure. As we stated earlier, pressure losses of only 5 psi (15 percent on a tire inflated to 32 psi) can dramatically affect the handling, the traction, and the durability of the average tire.


Are these three tires underinflated, overinflated or inflated to proper pressure? It is hard to tell the difference just looking at them. We underinflated the tire on the left by 5 psi, the center is at its proper pressure of 32 psi, and we overinflated the right tire by 5 psi.

Nitrogen for Tires


We normally inflated tires with a combination of gases comprised of Nitrogen (N2), Oxygen (O2) and Argon (Ar), commonly known as “compressed air.” Unfortunately, the compressed air we use to fill our tires contains moisture. Moisture reacts with the rubber compounds in the tire, causing them to break down and lose their strength and durability. This results in the natural leakage or permeation at a rate of about 1 psi per month we discussed earlier.

Oxygen also corrodes aluminum and steel wheel components and ages the thin layer of rubber inside the tire whose function is keeping air away from the carcass. As the thin layer of rubber or inner liner ages, more air molecules can pass through it, causing pressure losses.

More and more, driving aficionados along with a new crowd of environmental advocates are turning to nitrogen to fill their tires. Nitrogen gas is slower than air to leak from tires because the N02 molecule is larger, resulting in a more consistent tire pressure, cooler operating temperature, and longer tread life.  In addition, nitrogen does not oxidize tire components, reducing rim and wheel corrosion.

Michelin, BF Goodrich, Goodyear and many more tire manufacturers support nitrogen inflation for its ability to better retain pressure for a longer period of time.

Whether you fill your tires with compressed air or nitrogen, how frequently should you check tire pressure? Certainly any time the ambient temperature changes dramatically, but we recommend you follow the manufacturer’s maintenance intervals and specified inflation pressures. Get them in ChiltonDIY or ChiltonPRO.

A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 20 years experience in the automotive industry.