Winter Tire Pressure

This past weekend my tire pressure light in my car went on. I told a friend with me who was in the car who told me this was normal for cars in the winter and not to worry about the light. As I am from somewhere in California, where it is not often below 60 degrees F in the winter, I had no prior experience with this (this is my first year having my car here). This can be explained by physics by using the Guy-Lussac’s law – that P₁/T₁=P₂/T_2. Here volume remains the same, as no gas was lost, assuming there are no holes in any of my tires.

My tires were last filled to about 32 psi earlier this summer, so I will estimate the temperature to be 80 degrees F, or 300 K. 32 psi converts to 2.20 x 10⁵ Pa. This weekend when I noticed the light was on, it was about 28 degrees F or 271 K.

If we plug this into the equation we get:

(2.2 x 10⁵ Pa)/300K = P₂/271K

P₂ = 1.98 x 10⁵ Pa or 28.7 psi.

This decrease from 32 to 28.7 psi explains why the tire pressure light turned on.