How do you calibrate a tire pressure gauge?

[Uma]

Originally Posted by Sean

Bought a fancy-shmancy tire pressure gauge, and the pressures we settled on at the track last weekend seemed way low. Today I tested it and 3 others I had laying around on a tire: 33.5 for the "good" one, 37.5, 38, 39 from the three others I had. Uh...

So how do you calibrate these things? I guess I just need to figure out how to get one accurate, then I can calibrate the rest off that one.

All you have to do, is reverse the polarity.