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.

[tucsonjwt]

You should get 10-12 mpg city only.

A few suggestions:

Install manual choke - about $12. Then you will know when the choke is engaged and it should never be "stuck." Also, you can adjust the amount of choke you want - perhaps open it all the way shortly after a cold start if you are on icy pavement to avoid spinning wheels. I have found the most common reason for poor fuel economy is running rich due to a stuck choke, and this is a problem you can fix yourself for a little time and money.

Find an old mechanic who still has a SUN tune up machine. You can likely spend $40 for a half hour of diagnostics and get a good idea of the performance of your engine. You can also get an evaluation of the carburetor condition and assess whether or not you need a new one. I would avoid parts store rebuilds - if you need a carburetor get one from the professional Qjet rebuilders mentioned elsewhere on this forum. Modern mechanics don't want to be bothered with non-computer vehicles.

Drive like grandma on her way to church. Accelerate very slowly from a stop and take your foot off the gas long before you get to a red light or stop sign so you coast to a stop. If you are not irritating other drivers then you are not doing this right. If this improves your fuel economy then you know that driving style is a factor, then you can go back to driving any way you want if you think it is worth the gas $$$.

For reference, I have had 3 C20s (1973 454/TH400, 1976 350/TH350, and current 1983 454/SM465 granny) Of these, the 1973 got 11 city/13 hwy, the 1976 got 10 city/10 hwy, the 1983 gets 11 city/13 hwy (but it is currently only about 8 city due to excessive idling and being driven only 2-3 times per month.)