1974 Plymouth Valiant fuel economy?

I have a 1974 Valiant with terrible fuel economy. (10-13MPG) the engine i have in the car (225 Slant 6) is supposed to get just under 30MPG with an automatic transmission. The carburetor is rebuilt, i just replaced the PCV valve, spark plug wires, air filter, and spark plugs. i changed all the gaskets and hoses. what else would cause such poor fuel economy. what else should i check? I have a fair knowledge in auto mechanics, so don't be afraid to use technical terms.