Is 12 mpg attainable

[Dead Parrot]

What speed are you measuring at? My 75 gets 14 @ 60 and 12ish @70 and worse @ faster. If you are measuring around town mileage, all bets are off.

For the tune up, start with the ignition stuff. Check the plugs, wires, cap, you may need new ones. If you are non-emission or the check is post winter, move your vacuum advance to manifold vacuum. Verify that both your vacuum advance and centrifugal(mechanical) advance are working. It is fairly common for the vacuum pot to have failed at least once by now. Not uncommon for the mechanical advance to have stuck in one spot. Failure of one or both cause poor mileage.

If you still have stock exhaust, make sure the riser valve isn't stuck shut.

If still equipped with the OEM air cleaner, make sure the THERMAC system is working. Again, fairly common for the vacuum pot to fail which causes longer warmup times and worse mileage.

Is the choke working? A stuck or slow to open choke can really hurt mileage.

Have you done the Q-jet fuel well epoxy fix? Q-jets often leak there and hurt mileage. Have also had issues with leaks on Chevy Q-jets at the fuel filter casting plug. If you have a Olds Q-Jet, you may not have that plug.