Tire pressure changes constantly. Temperature swings of 10°F move PSI by about 1. Slow leaks bleed pressure over weeks. Pothole impacts can crack rim seals. Even healthy tires lose pressure naturally over time.
Check monthly with a quality gauge ($5 at any parts store). Pressures should match the door jamb sticker, not the maximum listed on the tire sidewall. Door jamb spec is the operational target.
Always measure when tires are cold — sitting at least 3 hours. Driving heats tires and increases pressure by 4–6 PSI. Measuring hot gives false readings and leads to chronic underinflation.
Underinflation causes uneven wear (outer edges wear faster), reduces fuel economy by up to 3%, increases stopping distance in rain, and can cause sidewall failure at highway speeds during heat waves.
Overinflation is less common but reduces traction and causes center-of-tread wear. The TPMS light comes on at significant pressure deviations but not until 25% below spec — well below the level that hurts performance.