[Synchro909]

When I got my first Model A, it was bog standard. On our club's economy test, I used to win regularly with a figure of about 27 miles per imperial gallon. 28 MPG is near enough, 10l/100km. On a rund, I'd always get about 26-7 and once, I topped 30 mpg (about 9.3 l/100km). All these figures were with an air cleaner attached.
I have since made a few modifications for more power and the economy has dropped to the low 20s mpg (say, 12-12.5l/100km).
It is important to note that we use the "proper" gallon (4.54 litres). The us gallon is only 3.8 litres. Even taking that into consideration, those figures suggest your car is too thirsty. In your position, I'd carefully rebuild a Zenith carburettor with flow tested jets. Most of the jets sold by the run of the mill vendors are too big. I suspect that might be your problem.