How do you untighten stuck car tire air caps that are?

For really stubborn stuck caps try the following methods.

For the first method you need a square inch of soft rubber like that from an inner tube of a Tyre and two pairs of pliers or mole grips. Wrap the soft inner tube rubber around valve below the cap then grab a firm hold onto the rubber wrapped valve with one of the pliers or mole grips. (The rubber helps with the grip and minimizes the chances of damaging the valve) Now take your second pair of pliers and attempt to unscrew the cap. It sometimes helps to get a friend to work one pair of pliers.

More stubborn are the aluminum Caps. These can almost weld themselves to the valve. If this is the case you may be able to remove these by cutting a grove down the sides of the cap then using a screwdriver try pry open the gap. Aluminum a very soft metal and can be manipulated quite easily. Unless great care is taken damage to the valve is likely so this method should only be used as a last resort.