Best possible mileage?

It is not just the AFR. A host of other factors come into play: car weight, gearing, component parts in the engine that affect cruise efficiency, etc, etc.
Two Pontiacs, one a 455 waggon that is over 4000lb gets 23 mpg on a trip with an 850 TQ; 2.56 axle. Another Ponty, 400 engine, 3.23 axle, about 3700 lb, gets 22mpg with an 850 TQ. These are Imperial gallons, so a bit less for US gallons, but impressive none the less.