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Tire pressure varies significantly with temperature because air takes up more volume at higher temperatures and less volume at lower temperatures. Being attentive to temperature when you check tire pressure will help you inflate your tires properly.
Recommended Pressure
Each vehicle has a recommended tire pressure noted in the owners manual and on the sticker typically found on the drivers door or door jamb. This recommendation is the tires pressure in psi when it is cold.
Temperature Effect
Tire pressure will change by one psi for approximately every 10 degrees of temperature change. Lower temperatures lead to lower tire pressure and higher temperatures to higher tire pressure.
Time Driving
In addition to varying with the outdoor temperature, tire pressure is affected by the tires heating up with use. During the first 20 minutes you drive your car, the tire pressure will increase by about one psi every five minutes.
Inflation
Tires should be inflated while they are cold, typically in the morning before you drive. If this is not possible, check the pressure and note how many more psi they need. Then check their pressure before you fill them and increase their psi by the amount noted in the morning.
Significance
Over-inflated tires are not in as much contact with the road and may cause a bumpy ride. Under-inflated tires cause unnecessary wear on the tire treads, loss of steering control, increased friction and lower gas mileage.
