Ford 2002 Explorer Sport Trac Owner's Manual page 14

Tires, Replacement Requirements
Each day before you drive, check
your tires. If one looks lower than
the others, use a tire gauge to check
pressure of all tires, and adjust if
required. Check tire pressure with a
tire gauge every few weeks
(including spare). Safe operation
requires tires that are neither
underinflated nor a vehicle which is
overloaded.
Periodically inspect the tire treads and remove stones, nails, glass or
other objects that may be wedged in the tread grooves. Check for holes
or cuts that may permit air leakage from the tire and make necessary
repairs.
Inspect the tire side walls for cuts, bruises and other damage. If internal
damage to the tire is suspected, have the tire demounted and inspected
in case it needs to be repaired or replaced.
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