Tire Pressure Indicators (set of 4), 34-38 psi One look tells you if it's time to add air. Replace your regular tire caps with these ingenious color-coded caps and avoid bending over and fiddling with a tire gauge to check the pressure. If air pressure drops more than 4 psi, the black cap changes to yellow, telling you it's time to add air. Proper tire inflation helps improve gas mileage, reduces tire wear and ensures a smoother ride. Set of four caps - one for each tire.