Low MPG

In addition to the above, you sure some kid isn't visiting you after dark?

You sure you don't have a leak in the fuel system?

Engine running up to temp?

Is your odometer correct? You sure you know how to "figgur" mileage? (Hey jus' askin')

FIRST thing I would do is play with the timing. I'm big on checking timing marks. This is because, "back in the day," I used to find a fair number --seemed like a lot of Frauds-- which had "slipped the balancer."

So buy or build a piston stop


Remove no1 plug, make sure the piston is "down a ways" unhook battery ground, and install the stop. Gently wrench the engine over until it stops "on the stop." Make a temporary mark on the balancer directly under TDC on the tab. Do the same thing CCW. Now you'll have two temporary marks some distance apart. True TDC will be halfway between.

While you are at it, gently wrench the engine CCW, the CW, "feeling" for the slop in the timing chain. With practice you'll feel it. You should have NO MORE than 10* movement at the balancer, otherwise you need a new cam drive (chain.)

Regardless of what the above shows, I would advance the timing and play with it, advance it until it "pings" and back it off some.

You absolutely sure the vacuum is actually working, and that the mechanical advance is not stuck?

This is HORRID mileage, and your 355's are NOT what is the problem.

A real life example:

Back when, my 70RR 440 sixpack, with headers, an 800 Holley DP, Edlebrock, 3.54, 4 speed, and "hang on" air, driven "not all that careful" would get just SHY of 14 mpg at 70 mph.

Later, that same car got a 71 high compression 340 dropped in. Headers, otherwise stock, it would pull more than 17 on a trip on the highway. 3000 RPM at 70mph

ANY stock 318 that is in tune in an A body should be able to get at least that good and up into the low 20's.

I didn't think it was possible to get that bad out of a 273