Tire pressure: how to find and set the right PSI

Federal safety standard FMVSS No. 110 requires every car, SUV, pickup and van up to 10,000 lb gross vehicle weight to carry a “Tire and Loading Information” placard. It goes on the driver’s-side B-pillar, the post you see when the driver’s door is open. Vehicles without a usable B-pillar put it on the edge of the driver’s door or the rear door instead. Some vehicles repeat the information inside the fuel door or glove box, and it is always in the owner’s manual.

The placard has to list:

The pressure on that placard isn’t a comfort setting for an empty car. The standard requires it to support the vehicle at its full rated load, so it is the right number for everyday driving with passengers and luggage.

The small print on the sidewall, often reading “max press 44 psi” or “51 psi”, is what FMVSS No. 139 calls the tire’s maximum permissible inflation pressure: the most air the tire should ever hold. NHTSA is explicit that it is not the vehicle maker’s recommended pressure, and fueleconomy.gov tells drivers not to use it.

The reason is that one tire size fits many vehicles. As Bridgestone puts it, the sidewall figure is a limit for the tire alone, while the vehicle maker’s figure accounts for that vehicle’s weight, ride and handling. The same 225/65R17 can call for different pressures on two different crossovers.

Placard pressures are cold pressures. NHTSA defines a cold tire as one that hasn’t been driven on for at least three hours, and tire makers including Bridgestone and Goodyear also count a tire driven less than about a mile. Driving warms the air inside quickly: research cited in NHTSA’s 2005 TPMS rule found tires generally reach a steady running temperature within about 15 minutes.

Warm tires read high. Kia’s owner’s manual says typically 4 to 6 psi above the cold figure. That leads to the most common pressure mistake: letting air out of a warm tire until it reads the placard number. Once the tire cools, it is under-inflated. Bridgestone, Michelin and Kia all warn against it.

If you have to add air after driving, NHTSA’s advice is to fill to the recommended cold pressure anyway. The tire may end up slightly low once it cools, but that is safer than driving on a significantly under-inflated tire. Bridgestone and Goodyear suggest setting warm tires about 4 psi above the cold number instead. Either way, recheck when the tires are cold.

Air contracts as it cools, so tire pressure falls with the outside temperature. Michelin and Bridgestone put it at about 1 psi for every 10°F drop, and Transport Canada at about 7 kPa for every 5°C. Some sources say 1 to 2 psi. The physics (the ideal gas law, worked with absolute pressure and temperature) explains the range: the higher the tire’s pressure, the bigger the swing.

The effect adds up over a season. A tire set to 35 psi on a 70°F autumn afternoon will read close to 31 psi on a 30°F winter morning without losing any air at all. Michelin notes that a tire set correctly in September can be several psi low by December. A tire set in a warm garage also reads lower once it is out in the cold, which Transport Canada flags for winter drivers.

Tires lose some air through the rubber, too. Tire makers estimate roughly 1 to 2 psi a month, though measured losses vary. That steady leak, plus a cold snap, is why so many TPMS lights come on during the first cold week of the year.

Every passenger car, SUV, pickup and van built since September 1, 2007 (up to 10,000 lb, excluding dual-rear-wheel trucks) has a tire pressure monitoring system under FMVSS No. 138. The rule requires the low-pressure light to come on within 20 minutes of any tire, or any combination of tires, dropping 25% below the placard pressure, or below a minimum floor, whichever is higher.

In other words, a car with a 35 psi placard can be almost 9 psi low on every tire before the light is required. NHTSA calls TPMS a warning that a tire is already significantly under-inflated, and the owner’s manual text the standard requires says the system is not a substitute for regular pressure checks.

Most systems are direct: a battery-powered sensor on the inside of each valve stem reports pressure by radio. A few are indirect, using the anti-lock brake sensors to spot a low tire spinning faster than the others. Direct sensors can need service when their batteries run down or when they are damaged during tire work.

TPMS does work. In NHTSA’s 2011 survey, 23.1% of 2004–2007 vehicles without TPMS had a tire at least 25% low, against 11.8% of same-age vehicles with it and 5.7% of 2008–2011 vehicles. NHTSA estimated the system cuts the chance of a severely under-inflated tire by 55.6%.

An under-inflated tire’s sidewall flexes more as it rolls. NHTSA’s evaluation explains the chain: more flex means worse fuel economy, duller handling, longer stopping distances and more heat, and heat is what leads to cracking, tread separation and blowouts. Under-inflation is also common. A 2001 NHTSA survey of 11,530 vehicles found 27% of cars and 33% of pickups, SUVs and vans had at least one tire 8 psi or more below the placard.

The crash data point the same way, with a caveat. In NHTSA’s pre-crash study, tires 25% or more under-inflated were about three times as likely to be cited as the critical event before a crash as correctly inflated tires, though NHTSA notes the records don’t prove the tire caused the crash. NHTSA reports 511 people died in tire-related crashes in 2024.

Fuel is the smaller benefit. Fueleconomy.gov says correct pressure improves mileage by 0.6% on average and up to 3%, and that you lose about 0.2% for every 1 psi drop in the average pressure of all four tires. Four tires each 5 psi low cost about 1%. Over-inflation isn’t the answer either: it shrinks the contact patch, reduces grip, wears the center of the tread and makes the tire easier to damage on potholes.

Buy a tire gauge and keep it in the car. Transport Canada and Michelin both say air-station gauges are less reliable than your own. The steps below combine NHTSA’s, the Tire Industry Association’s and tire makers’ instructions.

The placard lists front and rear separately because some vehicles specify different values. Keep each axle at its own number, and reset pressures after a tire rotation moves tires between axles.

Check pressure before carrying a heavy load or towing, and read your owner’s manual: pressures for special conditions such as heavy loads or high speeds are often found there rather than on the placard. Goodyear also notes that some van and light-truck makers list only the pressure needed at maximum load for Load Range D or E tires, which can over-inflate a lightly loaded vehicle. In that case Goodyear points to the owner’s manual or a load and inflation table.

The placard pressure was chosen for the original tires. If you’ve fitted a different size, XL tires where the original were standard load, or LT tires on a truck that came with passenger tires, the right pressure may be different. The owner’s manual text FMVSS No. 138 requires tells owners to determine the correct pressure for non-placard tires, and Goodyear says load and inflation tables are necessary for that. Ask the installer to work it out and label it. See changing tire size for more.

Many temporary spares run at 60 psi, far higher than the road tires, and federal rules require “Inflate to 420 kPa (60 psi)” to be molded on them. Check the spare monthly too, since you won’t notice it losing air until you need it.

Don’t let air out for traction in snow. Consumer Reports and AAA both advise against it: grip in snow comes from the tread’s edges, and low pressure hurts braking and handling. Use the placard or owner’s manual pressure, which may list a separate figure for winter tires, and never exceed the sidewall maximum. Transport Canada suggests letting tires stored on rims down to about 15 psi for the off-season.