Originally Posted by
dendawg
For whatever reason CO2 will leak out of a perfectly good tube over a few days
The reason is because CO₂ is more rubber-soluble than air. It literally dissolves faster into the rubber.
https://chemistry.stackexchange.com/a/54859
You don't
have to replace the CO₂ in a tire, as long as you pump your tires frequently. But if I were going on a very long day ride after using CO₂, I'd probably deflate the tire first.