At 3500psi, an HP100 has, well, 100 cubic' of gas while the LP85 has 113, although at 4000psi, the LP85 will have 129 cubes. Most dive shops won't jack up LP tanks much beyond 3000 (97) or 3200psi (103 cubic feet). I have a bunch of LP85's, and I never quite know what the the shop will settle...