Looks like this has been pretty well covered. (I just skimmed thru the last two pages.)
I can detect a difference of maybe 10 PSI by bumping the tires with a carpenter's hammer (my favorite) or my fist (in a pinch).
What I do is... listen to the tone -- the musical pitch.
Imagine you are the drummer in a band. For a higher note from a drum, you tighten the screws around the rim.
Same with a tire -- more air pressure, tighter skin.
When I use my fist, I do not bounce my hand off the tire as I do with the hammer, but rather I leave the hand pressed against the tire after the impact, and I FEEL the "musical" vibration.
Either way, I compare a pair of drive tires, and then I walk straight to the front tire while I still have the rear-tire tone in my head.
Then the other side likewise.
Always use steel valve-stem caps, and make sure the little gasket inside is in good condition. That cap is an integral part of the system that keeps you as safe as possible on the road.
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