The other day, my tire pressure monitor light lit up.

I checked the pressure of all 4 tires, and they were all at 28 psi. I normally fill them at 33 psi.

It has gotten colder recently. Does temperature affect tire pressure readings? Should I add air, or leave them as is (assuming that the PSIs will read lower than what they actually are)?

 

Short answer is cold weather causes a drop (theoretically 1 psi for 10 degrees) and you should refill. There's a couple of more detailed discussions going on in the coupe forum.

 

jgainnm is right on - the 1 psi per 10 degrees F is generally pretty accurate. The average tire can also lose 1 psi per month just from air bleeding slowly through microscopic pores and rim leaks. If you aren't on top of your pressures you could easily lose 3 or 4 pounds through the summer, and on a really cold fall morning find yourself down by 3 or 4 more. Pressures should always be set on a cold tire.