Idle mixture is precisely controlled because the idle adjustment screw is not metering raw, solid fuel past it. Before the fuel can even exit the idle metering slot it passes through an emulsion well, a small chamber in the bowels of the main body, that introduces air into the idle fuel stream through a series of precise orifii that breaks up the liquid fuel into smaller and smaller droplets. This emulsion passes by the metering screw where it is further regulated. Most high end carburetors (e.g. Webers and racing Holleys) have removeable adjustable air bleeds to further tailor the fuel delivery needs at idle. This is best done on a wet flow bench. Few of us have one of these in our garages so most production and aftermarket carbs settle for fixed air bleeds that are a best guess fit for most applications. I can tell you from my experience that this 'best guess' calibration is usually miles away from what a multiple-carbed inline six wants. Modifications to both the air bleed circuit and the idle transfer slots are imperative for the well tuned, multi-carbed hot rod six.
Anyway, back to the topic of gearing for fuel economy. Generally speaking, running the engine at a lower speed and at lower manifold vacuum (higher manifold absolute pressure) will produce BETTER fuel economy than spinning the engine higher at a lighter load setting. This is mainly for two reasons: 1) less "negative work" is done by removing the high vacuum from the tops of the piston during intake charging and
2) less frictional horsepower is lost due to the internal friction of the engine bearings, valve train, etc. Of course with a carbureted engine factors such the power valve / power enrichment circuit and optimal timing vis-a-vis engine octane tolerance at various loads also come into play. That is why the industry has gotten away from carbs and switched to EFI and has explored the fast-burn characteristics of combustion chambers to such a great extent.
There may be those who say, "progress was great at one time, but it went on for too long". We (the automotive engineers of the world) don't have all the answers yet, but we're sneaking up on it.
6re6
