Factors such as maintenance, temperatures, driving speeds, vehicle loading, and road conditions affect the wear rate of the tires.

Treadwear indicators are one way to tell when it is time for new tires.
Treadwear indicators appear when the tires have only 1.6mm (1/16 in) or less of tread remaining.
The rubber in tires ages over time.
This also applies to the spare tire, if the vehicle has one, even if it is never used. Multiple factors including temperatures, loading conditions, and inflation pressure maintenance affect how fast aging takes place. GM recommends that tires, including the spare if equipped, be replaced after six years, regardless of tread wear. The tire manufacturer date is the last four digits of the DOT Tire Identification Number (TIN) which is molded into one side of the tire sidewall. The first two digits represent the week (01–52) and the last two digits, the year. For example, the third week of the year 2010 would have a four-digit DOT date of 0310.
Vehicle Storage
Tires age when stored normally mounted on a parked vehicle. Park a vehicle that will be stored for at least a month in a cool, dry, clean area away from direct sunlight to slow aging. This area should be free of grease, gasoline, or other substances that can deteriorate rubber.
Parking for an extended period can cause flat spots on the tires that may result in vibrations while driving. When storing a vehicle for at least a month, remove the tires or raise the vehicle to reduce the weight from the tires.
Tire Rotation
Buying New TiresDanger, Warnings, and Cautions
Warning messages found on vehicle labels and in this manual describe hazards
and what to do to avoid or reduce them.
Danger indicates a hazard with a high level of risk which will result in serious
injury or death.
Warning or Caution indicates a hazard that could result in injury or death.
WA ...
Steering Gear Replacement (Hydraulic Power Steering)
Special Tools
EN 45059 Torque Angle Sensor Kit
For equivalent regional tools, refer to Special Tools.
Removal Procedure
Turn the front wheels to the straight forward position and secure the
steering wheel from moving.
Remove the lower steering intermediate shaft bolt (1).
...
Installation Procedure
Drill 8 mm (5/16 in) for plug welding along the edges of the service
panel as noted from the original panel.
Clean and prepare the attaching surfaces for welding.
Position the roof rear header (1) panel on the vehicle.
Verify the fit of the panel.
Clamp the roof rear header ...