A slow, unproductive, lazy person, student or worker that will hinder a group or team from their objective. Basically saying someone is inefficient in terms of miles per gallon, because a car that gets 12 to the gallon is not worth driving, in turn a person that is slow is not worth choosing or working.
A: Ay les put him on our team for the Government Project
B: Naw maine, he 12 to the gallon
A: Oh, he like that?
B: Yea, that fool hecka slow. We-a neva finish wit him messn' stuff up