First what Luc said.
To me, your question reads. I have a car I'm not getting 30mpg.
I don't know what car, I don't know if it's supposed to get 30mpg. I know nothing about your driving habits.
So, tell us...what kit? what were the specific gravities? how much did you top it up?
Best practices to get 12%? Buy a kit designed to produce 12% (some are more, some are less). Get a 6 US gallon carboy. Don't top up a lot (some won't agree on this one). Take specific gravity readings.
Yes you can add sugar to the primary if you want more alcohol.
Finally, as Luc suggested update the Location in the Control Panel, so that we know where you live. It can help with some questions.