You can fill gas canisters to full even if you're overencumbered

[ohgodspidersno]

You can carry thousands of pounds of gasoline if you load up your character to max encumbrance with empty fuel canisters and then have him fill them all at a gas station. The fueling does not stop when your true max weight is reached.

 

Not sure if this also holds true when siphoning gas, although I imagine it does.