Originally Posted by MikeWMass

I was thinking of this today when I pumped up my tires (which I had pumped up yesterday) Each time I put the pump on it reads about PSI below what I had pumped it to (pressure has been checked with a separate gauge, seems to be accurate after inflating). I would suggest you might want to pump up a tire to 110 or whatever, remove the pump, and then put it right back on and see what it reads. My guess about this is that the amount of air that is let out to fill the hose back to the gauge on the pump is enough to account for most if not all of the drop that I am seeing. Whether this is your problem I don't know, how big was the bottle you pressurized? Another way would be to inflate the tire and check with a different gauge.