2008 GMC Envoy Owner's Manual - Page 361

If your vehicle has P255/50R20 104V size tires installed on the rear axle, they will require inflation pressure adjustment when driving your vehicle at speeds above 99 mph (160 km/h). Set the cold inflation pressure, for the rear tires only, to the maximum inflation pressure shown on the tire sidewall, or 38 psi (262 kPa), whichever is lower. See the example following. When you end this high-speed driving, return the tires to the cold tire inflation pressure shown on the Tire and Loading Information label. See Loading Your Vehicle on page 4-34. Example: You will find the maximum load and inflation pressure molded on the tire's sidewall, in small letters, near the rim flange. It will read something like this: Maximum load 690 kg (1521 lbs) 300 kPa (44 psi) Max. Press. For this example, you would set the inflation pressure for high-speed driving at 38 psi (262 kPa). The inflation pressure for the front tires should be set to the cold tire inflation pressure shown on the Tire and Loading Information label. Racing or other competitive driving may affect the warranty coverage of your vehicle. See your warranty booklet for more information. Tire Pressure Monitor System The Tire Pressure Monitor System (TPMS) uses radio and sensor technology to check tire pressure levels. The TPMS sensors monitor the air pressure in your vehicle's tires and transmit tire pressure readings to a receiver located in the vehicle. Each tire, including the spare (if provided), should be checked monthly when cold and inflated to the inflation pressure recommended by the vehicle manufacturer on the vehicle placard or tire inflation pressure label. (If your vehicle has tires of a different size than the size indicated on the vehicle placard or tire inflation pressure label, you should determine the proper tire inflation pressure for those tires.) As an added safety feature, your vehicle has been equipped with a tire pressure monitoring system (TPMS) that illuminates a low tire pressure telltale when one or more of your tires is significantly under-inflated. Accordingly, when the low tire pressure telltale illuminates, you should stop and check your tires as soon as possible, and inflate them to the proper pressure. Driving on a significantly under-inflated tire causes the tire to overheat and can lead to tire failure. Under-inflation also reduces fuel efficiency and tire tread life, and may affect the vehicle's handling and stopping ability. 5-67