IIRC, the "leak" issue has to do with the relative partial pressures of each gas on each side of the Butyl barrier. Air is the same gas mix inside and out while CO2 is not. But I don't always IIRC well. ;-) I always just let most of the CO2 out when I get home, and pump it back up with air. Tires lose CO2 about twice as fast as they lose air, which isn't very fast. And, by the way, pressure is pressure. The CO2 holds you up just the same as the air does. And if you want to lose air fast, go with latex tubes!