FWIW, I have just had to use a CO2 cartridge roadside once, and it did seem to leak out quite a bit faster than air. The day after the CO2 infusion, I thought that I had pinched the tube! Pressure was down to something like 45 psi from probably 100+ psi. I pumped it back up to 120 psi using my floor pump, road it for about 15 miles and then let it sit for 2 days. When I measured it again, it was at 110 psi.