Miles per gallon?
These cars never got 18-22 mpg, even when new. 15 mpg on the high end and 12 is normal average in mixed driving. 10 mpg in the city is common. None of the more than dozen 225 slants I've owned ever got better than 15mpg, and that was only after looong trips at steady speeds.
Nonsense. My family owned 5 A-F body 225's when I was growing up, all bought new, and the '68 with auto trans got 18-22 mpg on the interstates, the 2 3 speed manual Sports were 20-23 mpg, the '76 Dart Lite OD manual got an honest 28 mpg and the last one, an '80 Aspen auto trans Super 6, got 24 mpg on the interstate. The in-town mpg was less of course, and a lot of cold winter starts will do that too, but not ever down in the 12-15 mpg range.
For the OP, I would first suspect that the choke is misadjusted to be 'on' more than it should. Also, the manifold heat riser may well be stuck closed and preventing proper heating of the intake. And then there is ignition timing, ignition performance, and a worn timing chain will hurt performance and mileage, by causing the cam to be retarded.
Some things that I would be working on:
CHOKE: With the car cold, work the throttle once and see if the choke plate snaps fully closed. After the car has run a few seconds look down the carb throat, and see that the choke plate is open about 1/8 of an inch. Afters about 5 minutes, see if the choek plate is open. If not, pull out the choke mechanism and look for a small nut and screwdriver adjustment with the markings 'R' and 'L' for rich and lean. Adjust a bit to L to see if the car is helped.
FLOAT LEVEL: This is very important to economy and driveability. Needs to be doublechecked.
HEAT RISER: This is a small rectangular flapper on the front of the exhaust right below the carb. (On the later engines, this is a round disc looking weight.) This should move freely at any time and temp. If not, use some penetrant and patience to slowly free this up; there is a penetrant/lube at the Mopar dealer for this. This should be closed when cold and as it heats, the bimetallic spring should move it open about 90 degrees.
IGNITION TIMING: Should be at least 5 advanced and some more is good. Suck on the hose to the vacuum advance and see if the timing moves more advanced. Check the points; the open gap should be around .018".
IGNITION PERFORMANCE: This is a long topic, more than I care to write up here, and it has all been said in other threads. One easy thing to check is the resistance of the ballast. The stock ballast shoudl be 0.5-0.6 ohms cold and < 2 ohms hot. Any thing higher than 1 ohm cold needs to be replaced as it will compromise the spark energy.
TIMING CHAIN: Very common to be worn and retard the cam 4-5 degrees. This needs to be examined with the timing cover off.
LEAKING GAS TANK: Self explanatory!
LEAKING FUEL PUMP: Smell the engine oil and if it smells anything like gas, put on a new pump, and change the oil IMMEDIATELY before driving again.
