The huge gas tanks shows the following stats when using Page Up and spawned with 100% level (set to air).
Pressure: 101.0442 ATM
Carbon Dioxide: 140.48 L
Oxygen: 73754.30 L
Nitrogen: 277316.19 L
Capacity: 3515.63 L
Tank Content: 351.210938
Tank Pressure: 101.044136
Stored Gas: -
Doing the math, the tank is filled to 9989.99% capacity but the tank reports only 10% filled. I haven't done the math on the other gas tanks, but they seem to suffer the same discrepancies, but scaled down with their respective sizes.