Well the caps came in today along with a shipment from Hong Kong FINALLY and some other stuff from Amazon. They are anodized aluminum and very well made. I ran pressure tests with them on the 90 psi air tank and an adjustable regulator connected to a valve stem I installed into a quick change air fitting months ago . . . how DID I know I would need one of those so long ago I can't say. They are basically a pressure gauge under a plastic cap (with a tiny hole in the center) and the plunger starts to move around 25 psi and pops all the way around 36-40 psi depending upon a little friction in the unit. Looks like they should give a great indication of tire pressure dropping. The ones I got are from Victor 00703-8 and unfortunately they don't seem to have 40 psi ones. Will install them on teh rear tires after the next big snow storm that is coming. They extend 3/4 of an inch past the tip of the stem threads so they should not be too much of a problem on my wheels with the stock hub caps. The racing disks would be a problem of course since you can't see throught them!