GMC 1994 Sonoma Owner's Manual page 271

When to Check
Check your tires once a month or more. Also, check the tire pressure of the
spare tire.
How to Check
Use a good quality pocket-type gage
to
check tire pressure. Simply looking
at the tires will not tell you the pressure, especially if you have radial tires
-
which may look properly inflated even if they're underinflated.
If your tires have valve caps, be sure to put them back on. They help prevent
leaks by keeping out dirt and moisture.
Special Tire Inflation
If you have 31xlOSOR15LT/C tires on your vehicle, and:
-
You'll be driving for very long at speeds of 66 mph (1 05 k d h ) to
75 mph (120 km/h) where legal, inflate tires to 10 psi (70 kPa) more
than the recommended cold inflation pressures.
- You'll be driving for very long at speeds of 76 mph (120 km/h) to
85 mph (135 km/h) where legal, cold inflation pressures should be
increased by 10 psi (70 kPa) and you must also reduce axle load
capacity by 10%. Do not drive over 85 mph (135 km/h) even when
it's legal.
Inflation pressures should never exceed 10 psi (70 kPa) above the
inflation specified for the maximum load of the tire.