Your tire is filled with gas, which in the cold doesn’t take up as much volume, so the pressure drops.
Some mornings you will wake up and you’ll have a couple pretty flat tires. This isn’t time to freak out! If you haven’t lost that much air, you can drive it to the closest gas station and put a couple quarters in and pump it up to the correct psi, or you can bring it to a garage where they will usually top it off for free. If this is happening very often, you might want to invest in a personal car tire pump, which run anywhere from $10-$100.
Never drive on an entirely flat tire, as you run the risk of ruining your rim and the tire completely.
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