What happens if you get a flat, repair it with CO2, then get another flat, but you only have one spare tube? How are you going to find the leak, if it's small, without a pump, with just that one spare?
If you have to carry a second spare, just so you can save weight with more CO2, then what's the point of CO2?
I carry CO2 for my road bike, a mini-pump, and one spare tube plus a patch kit. (On my fixie, I carry a more powerful, yet still small, Lyzene pump attached to the frame, a spare tube and a patch kit, and no CO2.)
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