340 Gets Terrible Gas Mileage

Hey guys, brand new to the forum. I just recently (about 30 days ago) bought a 1973 Dart Sport 340. It has a 340 with a performance cam, Edelbrock 650 4 barrel carb, headers with the Torqueflite automatic transmission. The first couple of tanks look like I am getting a little over 6 miles per gallon. This is all city driving but surely I should be getting much better gas mileage.

I'd appreciate comments on what mpg you think I should be getting and where should I start in trying to improve this.

Thanks

Dave
What's the timing curve look like?
What cam?
15 is possible if the cam isnt too hairy and the timing and curve is right.
Look for around 20 btdc initial & 34 mechanical, 48-52 total with vac advance hooked up.