Less gas is less gas, though! The long term cost is the real question.
As a cyclist, I have a perception of carbon as kinda fragile; take a good hit, and you really need to replace your fork. This makes me think the carbon will mainly be used for internal structure, which would still greatly reduce the weight of the plane (and this the base amount of fuel used), and bring down those lifetime costs. But if it's too fragile, then the overall lifespan of the plane is reduced, which would counterbalance the gains.
I had this impression for a long time as well, because it was true, but it may not be true any more. I can't pull it up right now (at work) but Santa Cruz did a great video comparing the strength of their carbon fiber frames to their aluminum frames, and in all scenarios carbon fiber frames proved the strongest. This included dropping weights on frames and forks, hitting them with hammers, etc... it changed my mind quite a bit about the resilience of carbon fiber.
As a cyclist, I have a perception of carbon as kinda fragile; take a good hit, and you really need to replace your fork. This makes me think the carbon will mainly be used for internal structure, which would still greatly reduce the weight of the plane (and this the base amount of fuel used), and bring down those lifetime costs. But if it's too fragile, then the overall lifespan of the plane is reduced, which would counterbalance the gains.