i get about 25 or 26mpg roughly, your wagon is HEAVY compared to mine though (62 2dr sedan, 200cid 3 speed auto). make sure its timed up, carb is set properly (no pinging under heavy load), tire pressure is even, ebrake isnt sticking, exhaust isnt stuffed or rotten or alot of bends, proper thrmostat and rad cap) over heating, under heating is the worse culprit) fuel filter isnt plugged, pvc valve is clean, air cleaner is good, list goes on....

first off id give a thorou tune-up (plugs,points,condensor, air filter,fuel filter, oil change, rad flush, adjust carb to best vaccumm), check brakes arent dragging, tire pressure. might be worth 5 or 6 more mpg if done right. but your wagon is heavy for such a small motor.

happy motoring