Use the vehicle's number—not the tire's maximum.
The correct starting pressure is listed by the vehicle manufacturer on the Tire and Loading Information label, usually on the driver's door edge or doorjamb, and in the owner's manual. The pressure molded into the tire sidewall is not the normal target for the vehicle; it describes the tire's maximum pressure under specified conditions.
Some vehicles specify different front and rear pressures. Replacement tires should still be set according to the vehicle placard unless a qualified source has provided a different specification for the exact application.
Check pressure when the tires are cold.
NHTSA defines a cold tire for this purpose as one that has not been driven for at least three hours. Driving warms the tire and raises its measured pressure, which can hide an underinflated condition. Check all four tires and the spare if the vehicle carries one that requires inflation.
A monthly check is a good baseline, with an additional check before a long trip and after a major weather change. A pothole, curb impact, nail or leaking valve can cause pressure loss at any time.
Modern radial tires can be significantly underinflated without appearing flat. Use an accurate gauge instead of visual judgment.
What the TPMS light is telling you.
A steady tire-pressure warning means at least one monitored tire is significantly underinflated. Check the tires promptly, look for visible damage and adjust to the vehicle's cold specification. If the light comes on during a cold morning and later turns off, pressure may be close to the warning threshold and should still be checked.
If the TPMS symbol flashes for roughly a minute and then remains on, the system may have detected a malfunction rather than simply low pressure. Sensor batteries, damaged sensors, wheel changes and system faults can all be involved.
Pressure is only part of winter tire safety.
Inspect tread and sidewalls for cuts, punctures, bulges, cracking or exposed material. New York's annual safety inspection requires at least 2/32 inch of tread in the measured grooves, but a tire at the legal minimum has much less ability to channel snow and water than a tire with deeper tread. Legal does not always mean well suited to the conditions you expect to drive in.
Winter tires are designed for cold, snow and ice performance in ways all-season tires are not. Whether they make sense depends on the vehicle, route, schedule and weather conditions. Correct size, load rating and installation on all recommended positions matter.
Avoid the overcorrection.
Do not intentionally overinflate tires to compensate for future cold weather. Set the pressure to the placard specification while the tires are cold and recheck as conditions change. Too much pressure changes the contact patch and can affect ride, wear and traction.
When a low tire needs more than air.
If one tire repeatedly loses pressure, falls much lower than the others, has sidewall damage, or was driven while nearly flat, it needs inspection. Adding air repeatedly does not repair a puncture or reverse internal damage caused by underinflated driving.
This guide provides general tire-care information. Follow the specifications in the vehicle owner's manual and tire placard, and have damaged or repeatedly low tires inspected by a qualified professional.