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The old saying “where the rubber meets the road” usually means something will reach a critical point. Naturally the expression refers to automobile tires, since those crucial pieces of rubber are the only part of your car or truck that are supposed to make constant contact with the pavement. That’s why it’s important to know and maintain the correct amount of air pressure in your vehicle’s tires.
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To get the right amount, turn not to the tire itself but to your vehicle and a label often located in its doorjamb. That’s where you can find the vehicle manufacturer’s suggested inflation level for your car or truck. If it’s not there, check the owner’s manual. Recently, a reader asked why tire sidewalls often show a pressure rating, shown in maximum pound force per square inch (psi), that differs from the vehicle maker’s recommendation. This may be obvious to racing enthusiasts and mechanics, but not all motorists understand all the nuances. So we turned to some of the experts.
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Tiremakers start with a set of standards, set by the Tire and Rim Association (TRA) in the United States and the European Tyre and Rim Technical Organisation (ETRTO) across the pond, and use their guidelines in developing tire sizes’ maximum psi and maximum load levels.
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“There’s a load/inflation table for every size of tire in the universe,” said Bob Toth, director of industry relations at Goodyear. But cars and trucks all have varying weights and sizes, and they handle and perform uniquely, Toth noted. Vehicle manufacturers come up with ideal tire pressures based on factors such as how much weight a car or truck puts on its left front wheel during a right turn at 60 mph, for example. Some cars and trucks wind up with different recommended pressure levels for front and rear tires, even on all-wheel-drive vehicles.
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Here’s an example: On the tires on a Lexus LC500 we have at the C/D offices just now (pictured above), you can read a maximum pressure of 51 psi, but the automaker recommends 33 psi (shown on the sticker below) for both front and rear tires. Sarah Robinson, a driving-safety expert at Michelin, said the manufacturer’s recommended tire-pressure level dates back to the beginning of a vehicle’s development process. Tire engineers from companies such as Michelin will work alongside automakers’ engineers using the TRA and ETRTO guidelines. “You’re going to be tuning the pressures to fit that envelope of air that those tires allow,” Robinson said. “So max pressure is precisely that, but within that, there is the ideal pressure to maximize the performance of the vehicle.”
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Through rigorous testing, the vehicle’s recommended tire pressure is determined, with the idea of balancing considerations of performance, fuel efficiency, safety, and overall longevity.
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But how important is that number? Should it be tweaked for certain scenarios? For most consumers who are not heading to a racetrack or following guidelines for pulling a trailer, the answer is absolutely not. “I cannot emphasize enough the importance of proper tire inflation pressure,” said Keith Willcome, sales engineering manager for the Americas at Bridgestone. “Inflation pressure affects every aspect of the tire, including the load carrying capacity, tread contact patch shape—where the tire touches the road—and size and handling characteristics.”
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If a tire is underinflated, more heat is generated inside of it, and this could cause structural damage, Willcome said. An underinflated tire also cannot absorb shock as well, and so it is more vulnerable to road hazards such as potholes. What’s more, the tire’s tread footprint—where the rubber meets the road—will not be its correct shape. This puts more wear on the tire’s shoulders, those rounded parts of the tire between the sidewall and the tread. The tire will also likely have less grip if underinflated, Willcome said.
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An overinflated tire can have less grip, too. And it can affect handling, because the overinflated tire is not correctly absorbing impacts, which can send more force to the vehicle’s suspension. Overinflation also makes the tires’ tread footprint smaller, which can increase wear on the center of the tread.
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The key to maintaining proper tire pressure is checking psi levels at least once a month, according to the tire companies we talked to and to the National Highway Traffic Safety Administration (NHTSA). But who actually does that? Not many people. NHTSA estimates that just 19 percent of consumers properly check and inflate their tires, even though tires tend to deflate by about 1 psi a month. About a quarter of all cars have at least one tire that is significantly underinflated, according to the agency. If nothing else, NHTSA noted that keeping tires properly inflated can help save as much as 11 cents a gallon on gas.
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Checking the Pressure
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One quick and easy way to check tire pressure is with a handheld tire-pressure gauge, which typically costs less than $ 10 at an auto-parts store. You just unscrew the tire-valve cap, plug in the gauge, and wait for a reading. If you don’t want to mess with that, tire shops will often check the pressure for you, and some gas stations have digital readouts as part of their air pumps. It’s best to check the tires when they’re cold, meaning that they have not been driven on for several hours.
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Newer cars and trucks also have tire-pressure monitoring systems, and some report current tire pressure directly to the driver’s instrument panel. But many such systems don’t notify the driver until tire pressure is far too low, with a warning light that doesn’t come on until the tire is already 25 percent underinflated.
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“When that warning light comes on, that means either that you have a rapid leak or you’ve been driving along gosh knows how long with an underinflated tire,” Toth said.
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