noob needs help with mileage - only 10 MPG!

I've got a 1966 Valiant with a 225 slant six and the Torqueflite 3-speed auto. The engine is not original, and according to the casting numbers it is a 1980 production year engine. Anyway, I've had this car only about 2 months, its my first Mopar, my first slant six, and my first classic car. So I'm truly a noob here.

But 10 MPG!?!?
That's with a decent mix of in town and highway driving. I was expecting mileage in the 20 MPG ballpark.

The car runs great, so I'm not sure where to begin troubleshooting this. My guess is the carburetor is key to this lousy fuel consumption, but that's only a guess. The car starts up fine with the engine cold, but once the engine is warmed up it does idle rough. Other than that, there are no problems with drive-ability.

And I've been putting premium gas in the tank...

I'd greatly appreciate any help or hints here.
Thanks, guys.