Tire pressure can decrease about 1 PSI (pound per square inch) for every 10 degrees the temperature drops. It’s not that more air is escaping your tires, but rather the air inside the tire condenses, taking up less space when it’s cold. It’s similar to how a cake, just out of the oven, flattens out a bit as it cools.

Tires also lose about 1 PSI per month just from seepage of air around the edge of the rim and through the tread itself.

These two factors combined can cause the air pressure in a tire to go 25 percent below the recommended fill pressure. This is what triggers the sensing transmitters inside your tires to illuminate your TPMS dash light. Whenever
your TPMS light comes on, check the air in your tires and bring back up to the proper pressure.

Winter Tire Pressure
Temperature changes outside affect your tire pressure. If it gets up to 45 degrees by day and drops to 15 degrees or less at night, your tire pressure can vary 3 PSI, not counting normal air loss. Which is why it’s not unusual to have the low-pressure indicator light go on first thing in the morning, since it’s usually coldest overnight.

The light may shut off on its own after you drive 20 minutes or so, as the air in your tires warms and expands and proper inflation level stabilizes.

Regardless, you should get your air checked right away. The TPMS light means your tires are at least 25 percent below the proper air pressure. Which can be a safety risk, especially if you’re carrying a load close to your vehicle’s max capacity. There’s a greater chance of tire failure, compromised handling and increased wear and tear on your tires. Your gas mileage could also suffer, and at today’s prices you definitely want to monitor that.

As the tire regains proper pressure the TPMS light will go out.

Note: If the warning light is flashing, this is a problem with the vehicle’s TPMS system, not your tires, and you should bring your car to the dealership.

One More Reason Your TPMS Light May Go On
Your TPMS light may flash if your vehicle’s onboard computer can’t detect the sensor because you’re using a spare tire. They typically don’t have TPMS sensors or if you have winter tires on that also don’t have the TPMS sensors installed.

How to Get Winter Tire Pressure Right
Once a month, check your pressure when the tires are cold (meaning the car is parked outside and hasn’t been driven in at least four hours) inflate them to what’s indicated on your placard located on the inside of your vehicle door.

For more information regarding TPMS with your vehicle, please review your Toyota owner’s manual