-- Tire pressures should be checked at least weekly, and at the same time should be checked for damage, cuts, bulges, and embedded objects.

-- The CORRECT pressures are those provided by the vehicle manufacturer - usually on a sticker which will either be on the driver's door or door frame (visible only when the door is open), or in the engine compartment. The pressure shown on the wall of the actual tire is only the maximum permissible pressure at which that tire may be used; it is almost certainly NOT the "correct" pressure for use on your vehicle!

-- Depending on your country, the minimum, legal tire tread depth may be 1/16 of an inch, 1.6mm, or whatever. But if safety is your main concern it is important to change the tires while they still have double the above amounts of tread depth remaining. This is because the grooves serve to clear water away on wet roads and the more efficiently they can do this the safer you are, period! And for use in snow, three times the above tread depths should be considered the minimum for safety.