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If hybrid buyers were interested in reducing CO2 emissions at the lowest cost, they would simply be buying carbon credits online.
So clearly there are other forces at work. Signalling to others about environmental responsibility is a reasonable thing to do, and you can hope the social pressure (and manufacturer response to demand) will have some effect.
But for an affordable solution which also sends the right pro-environment message, I'm partial to my TerraPass (www.terrapass.com).