PSI (pounds per square inch) is the unit that measures the air pressure inside your tires. To set it correctly, measure each tire when it is cold with a reliable gauge, then inflate or release air until it matches the cold pressure printed on your vehicle’s tire placard, not the number molded into the tire sidewall.
How to Check and Set Your Tire PSI
NHTSA defines a cold tire as one that has not been driven for at least three hours. If you have to drive to an air pump, pick one close to home and check the pressure there, because a short trip warms the tires and raises the reading slightly.
What you need
- A tire pressure gauge (digital or dial; a gauge built into a cheap pump is often less reliable)
- An air compressor or access to a station air pump
- Your owner’s manual or the tire placard for the recommended pressures
Steps
- Find the recommended cold pressure. Look at the Tire and Loading Information label on the driver’s door edge or door jamb (B-pillar), or in the owner’s manual. Write down the front and rear values.
- Remove the valve cap from the first tire.
- Press the gauge straight onto the valve stem firmly until the hissing stops, then read the value.
- Add air in short bursts if the reading is low, and recheck after each burst.
- Release air if the reading is high by pressing the small pin in the center of the valve stem for a second or two, then recheck.
- Refit the valve cap to keep dirt and moisture out of the valve.
- Repeat on all four tires and, if your vehicle has a full-size spare, on the spare as well.
NHTSA advises checking tire pressure at least once a month when the tires are cold, and before long trips or when carrying heavy loads.
Where to Find the Right PSI
The correct pressure is the one the vehicle manufacturer lists. The placard or manual may give different values for front and rear tires, and sometimes different values for normal and fully loaded conditions. NHTSA is explicit that the correct pressure is what the vehicle manufacturer lists, not what is printed on the tire itself.
The number on the tire sidewall, usually marked “Max Press”, is the highest pressure the tire is built to hold. It is a limit, not a target. Filling to it on a typical passenger car usually means a harsher ride and a smaller contact patch.
| Where you see a number | What it means | Use it to inflate? |
|---|---|---|
| Driver’s door jamb placard | Manufacturer’s recommended cold pressure for the original tire size | Yes |
| Owner’s manual | Same recommendation, sometimes with load or speed notes | Yes |
| Tire sidewall “Max Press” | Maximum pressure the tire can hold | No |
| Gauge or pump reading right after driving | Warm pressure, higher than the cold value | Only as a rough check |
If you have fitted a different tire size than the original, the placard may no longer match. In that case, ask the tire shop or the tire maker for the correct pressure for your size and load.
Why the Right Pressure Matters
Underinflated tires flex more, run hotter, wear faster along the edges and respond more slowly when you steer. Overinflated tires have less rubber on the road and wear faster in the center.
Pressure also affects fuel use. FuelEconomy.gov (DOE/EPA) says you can improve gas mileage by 0.6% on average, and up to 3% in some cases, by keeping tires inflated to the proper pressure. It also says underinflated tires can lower gas mileage by about 0.2% for every 1 psi drop in the average pressure of all tires.
Convert PSI to Bar and kPa
Some gauges, pumps and placards use bar or kPa. Use these conversions:
- PSI to bar: divide by 14.5038 (35 PSI is about 2.41 bar)
- PSI to kPa: multiply by 6.8948 (35 PSI is about 241 kPa)
| PSI | Bar (approx.) | kPa (approx.) |
|---|---|---|
| 30 | 2.07 | 207 |
| 32 | 2.21 | 221 |
| 35 | 2.41 | 241 |
| 40 | 2.76 | 276 |
For official guidance on tire pressure and placards, see NHTSA tire safety information.
Common Mistakes
Using the sidewall number
Inflating to the “Max Press” value ignores what the vehicle maker calculated for your car’s weight and handling. Use the placard.
Checking right after driving
Driving warms the tires and raises the pressure. NHTSA notes that warm tires read higher than cold ones. If you must add air when the tires are warm, add a small amount, then recheck the next morning against the cold value. Do not release air from warm tires to reach the cold number, or they will be underinflated once they cool.
Ignoring temperature swings
Air pressure drops as the temperature falls. A commonly used rule of thumb is about 1 PSI for every 10°F change in air temperature. After the first cold snap of the season, recheck all four tires, because a drop of several PSI overnight is normal and may trigger the warning light.
Forgetting the spare
NHTSA notes that spares age because they are seldom replaced. Check yours when you check the others, since a flat spare is no help on the roadside. Some compact spares call for a much higher pressure than your regular tires, so read the value printed on the spare or in the manual.
TPMS Is a Warning, Not a Gauge
Vehicles sold in the US since the 2008 model year have a tire pressure monitoring system (TPMS). Under the federal standard, the warning light comes on when a tire is significantly underinflated, at 25% or more below the placard pressure. That is a large drop: on a 35 PSI tire, the light would come on at roughly 26 PSI or less. A tire can be 5 or 6 PSI low and still not trigger it.
- If the light comes on, check all four tires with a gauge as soon as it is safe to do so.
- If the light flashes for about a minute and then stays on, many vehicles use that pattern to signal a system fault. Check your owner’s manual.
- Some vehicles require a reset procedure after you adjust pressure. The steps vary by make and model year, so follow your manual.
When to Have a Tire Inspected
- One tire keeps losing air while the others hold steady, which points to a puncture, a bad valve stem or a wheel-rim leak.
- You see a bulge, a cut, an embedded nail or cracked sidewall rubber.
- The vehicle pulls to one side or vibrates at speed after you have set the correct pressure.
A tire with a visible bulge or sidewall damage should not be driven on at higher speeds until a tire shop has looked at it.
Frequently Asked Questions
What should my tire PSI be?
Use the cold pressure on the door-jamb placard or in your owner’s manual. Many passenger cars call for pressures in the low-to-mid 30s PSI, but trucks, SUVs and some cars differ, so the placard for your vehicle is the only number that counts.
What does 35 PSI mean?
It means the air in the tire presses on each square inch of the inside surface with 35 pounds of force. Whether 35 is right depends on the placard for your vehicle.
Is 40 PSI too high?
If your placard says 32 or 35 PSI, then 40 PSI is over the recommendation and can give a firmer ride and less grip on wet roads. If your placard or manual specifies 40 PSI or more, for example for a heavily loaded vehicle or a specific truck, then that is the correct value.
Should I inflate to the same pressure in winter and summer?
Yes, the target cold pressure stays the same. The air inside simply reads lower in cold weather, so you may need to add air more often in winter.
How often should I check my tires?
At least once a month when they are cold, and before long trips. NHTSA recommends this monthly check even if your car has TPMS.
For more on the fuel economy side of tire care, see FuelEconomy.gov’s tire inflation guidance.
