6 banger mileage?

ok I know this might be a really dumb question but would it matter what size Carb you have? I just got a 1965 dart for my son and we were wondering about gas milage. I told him as long as he is getting good grades I would buy him one tank of gas a week... after that it was up to him and his friends... he asked me what kind of gas milage it got and i told him it should be really good. but hell honestly i do not know.
its a charger 225 not that I think the "charger" means anything with a 1 brl carb. My brother got a rebuild kit for the carb and it fired right up. So i am seeing here just as an average 15-18 city if its in decient shape? but would the carb make a difference?