Spider210

On my civic which has been getting horrible gas mileage btw...(15mpg or about 155 miles for 10.6 gallons of gas) my alternator is not working correctly. It squeaks sometimes when I first start it in the morning for about 2 minutes then it stops. I had it tested just few days ago at auto zone and a stock alt. pushes 14.4volts and 70amps. Well my alt. was pushing 12.1volts and 70amps. What can cause this all of the sudden? I think this has to be the problem for my poor gas mileage. Is there a way to adjust the voltage coming out? I checked the fuse that is #15 for the alternator but that was fine. The voltage does not change when its idling nor when its reved up to like 4k.