What's amusing are the customer reviews for this pressure gauge at Amazon.com, where a ten pack runs for $37.
1. "At 50 cents a piece, [the pressure gauges are] hard to resist. At this price, I don't mind giving them away to friends and family." Definitely not a math major.
2. "Are they 100% accurate? I don't know, I don't have a calibration kit. Do my tires feel really firm when the gauge says 65psi? Yes. That was my way of 'calibrating' them." If gauging tire pressure by feel works well enough for you, why bother with the pressure gauge?
Do you use a small tire gauge for your bike tires? I use the pressure gauge built into the floor pump.