Tire manufacturers suggest checking tires when they're cold for the most accurate reading. Outside temperatures can cause tire pressure to vary by as much as 1 psi per 10 degrees; higher temperatures mean higher psi readings. "Tires are black; what does black do? Attract heat," Rastetter said, noting the importance of finding a shady place to check and fill all four tires.
Temperature plays a huge part in tire psi, Rastetter said, adding that the most crucial time of year to check pressure is in fall and winter when days are shorter and average temperatures plummet.
Check your tires in the morning before going anywhere, because as soon as you get behind the wheel for an extended amount of time, psi will rise. Rastetter said that if you've been on the road a long time and notice higher psi in your tires, don't let the air out, as the increase in pressure has built up due to the warm, constantly-in-motion tires