In the days before cars came equipped with a Tire Pressure Monitoring System (TPMS), correct tire pressure was often the most widely ignored item on a car’s maintenance checklist. Worse, when it was adjusted, it was frequently checked when tires were warm, since few motorists went to the trouble of buying a compressor (and an accurate tire gauge) for home use.
Improper tire pressure can affect everything from fuel mileage to vehicle safety, as incorrectly inflated tires can produce unusual (and in some cases, unpredictable) handling. Checking tire pressure on a regular basis (every two weeks is recommended) also ensures that tires aren’t punctured, in need of rotation, or worn beyond safe and permissible levels.
To ensure accurate readings, each tire should be checked cold, preferably in the shade or in an enclosed garage (to avoid the increase in tire pressure potentially caused by direct sunlight). If you’ve been driving the car, allow three or more hours for the tires to cool before checking and setting tire pressure. As a baseline, the tire pressure recommended by the manufacturer for your vehicle, on stock tires, can generally be found inside the driver’s door, on a sticker affixed to the bottom of the B-pillar.
In front-wheel-drive vehicles, the front tires generally carry a higher inflation pressure than the rear tires, as the bulk of the vehicle’s weight rests on the two front tires. The opposite would be true for rear-engine vehicles, which carry the most weight atop the rear tires; in any case, be sure to note differences in inflation pressure from front to back as recommended by your vehicle manufacturer.
Sometimes, this pressure will vary by load, as well, with manufacturers recommending higher tire pressures for heavier loads. Under normal driving circumstances, tire pressure should always be set to the value that most closely matches your current vehicle loading (which is another reason why tire pressures are not a set-and-forget maintenance item).
Always use a quality tire gauge to read the current inflation pressure of each individual tire, including the oft-forgotten spare. There is much debate on what constitutes a “quality” tire gauge, but those who race cars (and view proper tire pressure as the difference between a good lap time and a back-of-the-pack lap time) generally prefer the dial-type gauges, equipped with a built-in valve to bleed off excess pressure. As with any other tool, air pressure gauges are generally worth what you pay for them, and given the importance of proper tire pressure, getting by with a free or inexpensive tire pressure gauge seems like a bad place to cut corners.
Once the correct tire pressure has been set at each corner, using a portable compressor or air tank, it’s time to make a few observations. First, did any tire differ significantly in pressure from its recommended value? If a pressure in one corner was lower than the tire on the opposite side of the vehicle (comparing front to front and rear to rear), chances are the tire has an air leak. Inspect the tread for foreign objects like nails or screws; if none is found, use a dish soap and water solution to inspect for leaks around the valve stem. Squirt or pour some of the solution around the base of the valve stem, then look for bubbles; repeat for the valve mechanism itself. Any punctures or leaks should be addressed by a competent shop, as not all punctures can be repaired.
While this advice applies to stock (or equivalent) tires under normal driving conditions, there are exceptions to the rule. Switching from an all-season radial to a high-performance summer tire will generally necessitate a change in tire pressure, but finding the optimal pressure can be a matter of trial and error. The old autocross trick is to start at the manufacturer’s recommended pressure, then use a bit of white shoe polish to mark the tire’s shoulder and sidewall. After a run through the course, look at how far the tire is rolling over into the shoe polish; if the tire is scrubbing into the sidewall, add a few pounds of pressure and repeat, making sure to log all changes on your setup sheet.
Manufacturers’ pressures are also designed to deliver a blend of comfort and handling; if a firmer ride is an acceptable trade-off for (slightly) improved handling, it’s generally permissible to up tire pressure by a pound or two from the published numbers. On the other hand, the maximum inflation pressure (shown on the tire’s sidewall) should never be used; not only will this deliver a harsh ride and unpredictable handling, but the tire’s pressure can potentially rise to dangerous levels as the tire heats up.
Finally, many tire dealers and automotive dealerships are now touting the benefits of filling tires with nitrogen instead of air (which itself is composed of 78-percent nitrogen, 21-percent oxygen and traces of carbon dioxide and water vapor, according to NASA). If this is a free service, nitrogen will deliver a few minor benefits; first, pure nitrogen seeps from tires more slowly than air, meaning that tires will maintain pressure longer. Nitrogen expands less as it heats, meaning that tires will maintain more consistent pressures, even when hot. In the world of professional racing, where victories can be determined by thousandths of a second, such benefits are worth the added cost of nitrogen. In the real world of commuter driving, the benefits are only worthwhile if the use of nitrogen is a free service offered by your dealer.