I guess the first question is: how bad is "pretty bad?"
Edit: never mind...tired and forgot the title of the thread "why am I only getting 6 mpg?" Most of the rest applies though. Don't overlook dragging brakes, or similar mechanically induced resistance.
Having asked that, allow me a brief moment on the soapbox, whereupon I opine that due to the way gasoline is made, as well as the way the ugly companies who manufacture gasoline are made, there is only so much gas mileage that you can get out of a gallon of gas...unlike back in the day. In other words, either due to the makeup of petroleum distillates, as well as the additives therein, or due to the corporations themselves, we're only going to get less than optimal gas mileage while the corpor-rats get more than optimal gas profits.
--end of rant--
Do you know any old mechanics in your area who have access to a 2- or 4-gas analyzer? A 2- or 4-gas analyzer can tell pretty quick what's coming out of the tail pipe, unburned, or partially-burned Hydrocarbons wise. Especially since you don't have a catalytic convertor to cover up potential problems in your engine.
There is a possibility that your carb may be dumping fuel into the intake manifold, but may not be showing up as rough running. Dumping will show up on the analyzer as high CO, whereas partially burned fuel (generally from a misfire of some kind) will show up more as high HC.
Let's play "let's pretend." Let's pretend that you have an intake manifold leak. Normally, that'd cause a misfire and the engine would run rough, particularly at idle. Now, let's say your carb was dumping fuel at the same time. Since older cars with higher mileage generally tended to run smoother with a richer-than normal air/fuel ratio, the dumping fuel (high float, leaking gasket, accelerator pump leaking), could actually mask a lean condition like a leaking intake manifold, causing you to believe that according to the way it's running, all is well. Edit: well, except for the old 6 mpg thing.
The vacuum advance could contribute to poor fuel economy, though I wouldn't expect to see hugely bad numbers just out of a bad advance.
Your test is a good test for the manual part of the advance test...are the flyweights mechanically advancing the timing is what it checks. It's also a good test at a high rpm, when your manifold vacuum comes back, but not from off idle, where your manifold vacuum goes away. Not sure if your vehicle controls vacuum via the manifold or venturi on the carb, but I'm guessing on the manifold.
Anyway, if you don't see any advance when you rev the engine, you've got a problem with your distributor, and it will need to be serviced with a kit, even overhauled, or replaced, if need be. You should see (guessing here) 4 to 6 degrees of advance through the flyweights alone. If you have venturi (carb) vacuum, you will see several more degrees of advance off idle, whereas manifold vacuum only advances the timing at idle and under cruise conditions (when the throttle plate is more closed than open).
Having said all that (minus the rant), it absolutely could be a combination of things, but I wouldn't think your gear ratio would contribute that much to the problem, again, depending upon how bad "pretty bad" is.
Edit: per my remarks at the beginning, I see how bad "pretty bad" is.
