This item is well made of for durable and practical use. Green/yellow/red system indicate level of proper tire inflation. According the recognize color to display the pressure of the tire,Green shows that tire pressure is normal Yellow shows that the pressure is 10% lower than standard you that the pressure is 25% lower than standard. Feature - Color: As shown. - Material: Alloy and PC. - Size: Approx. 1 x 1 x 2.5 cm / 0.4 x 0.4 x 1.0 inch ( W x H). - Visually alerts you when the tire pressure is low. - Help to avoid premature tire wear or blowouts from under-inflated tires,increases fuel efficiency, safety and control, extends tire life. - Green/yellow/red system indicate level of proper tire inflation.According the recognize color to display the pressure of the tire, Green shows that tire pressure is normal Yellow shows that the pressure is 10% lower than standard, you that the pressure is 25% lower than standard. - Applicable to car,motorcycle,bicycle and so on. Package list: 1 x set of 4 PCS Car Tyre Caps Covers Tire Pressure Monitor Stem Caps Sensor Indicator