Bought this to go in my pack. Doesn't read. Reads my front tire, but doesn't read my back. Looks like the center piece is flush with the part that contacts the top rim of the valve. If your valve stem is flush or below the rim it won't work. Crap. Buy something else.
EDIT: My air pressure in the rear tire was low, about 4 lbs. If I increase the pressure in the tire the gauge will read sporadically. Two other gauges, one digital and one other cheap gauge I got from the auto parts store (0-20 psi) read this tire every time and both read it as 4 psi.