there is a long list of things that can cause poor milage as listed before.. a dirty air filter, draging a/c or alt can bog engine down. timing can be off. i got a free 5th ave from my father in law and it got maybe 5 mpg. give it a good oil change and change plugs, i use champions gapped at .045 and check your timing, you cant always go by the timing marks on old engines, i adjusted timing by advanceing a bit at a time till it ran the best with out pinging-knocking. i found that my engine likes the timing advanced far enough that the housing is turned fully advanced ,it cant turn any farther. does your 318 have leanburn? if so ,maybe there is problems with the puter and or carb-dist. best thing to do is check everything that you can do to your car to optimize the running, brakes-driveline- transmission, change fluid and filter not forgetting to adjust bands to factory service manual specs. my car now gets 12 to 14 on the hiway, could be better but ill give her more care as soon as winter gets done freezing us out,lol