I measured the PSI on my rear driver tire (using a gauge pin - push it into the needle and a stick pops out with a reading), and I got a reading of 22 PSI. My tires are supposed to be anywhere from 30-34 PSI. The thing is, the rear driver tire (the tire in question) doesn't look flat at all. It looks just as good, if not better than the other tires.

Should I fill the tire with air? I don't want to over-inflate the tire, if my gauge is giving me a bad reading. However, at the same time, the gauge seems to be working fine for the other tires, so one would be reasonable to assume that it's working well for the tire in question as well.

Another note to consider - my front driver side tire looks flatter than the rear driver side tire, but it's PSI is 32.

What should I do? Should I go with how the tire "looks" or what my $4 (from Advanced Auto) gauge says?