[77newell]

Tires flex going down the road. The flexing generates heat. The softer the tire is and the faster it rotates the more it flexes and the hotter it gets. I normally see 10-15 psi rise during driving. At home I've seen 5-10 psi difference between sunny side and shaded side. On the road my inner dual tires run warmer than outers. So, even if I were to set all my tires the same in a shaded parking spot as soon as I drive in sun at speed the pressures go all kittywampus (that's a tire technical term). The manufacturers know this and this is why we measure them before driving.