I don't seem to have any punctures (touch wood!) and I can usually squeeze another 20psi into the tyres when I pump them. A few psi will be from me pressing the vale when I attach the pump head, but not the 20, as there's a discernible difference in firmness in tyre feel before and after.
Do all (most) valves leak slightly? Is the tube porous (I doubt it)? What causes this phenomena?
It's no drama, I just keep an eye on my tyres and inflate accordingly. Just interested is all
