Into the weeds engine design for fuel efficiency discussion.

Dale Davies said

Exactly. Now if we look at engine displacement and watch the carburetor. Holley has a power valve that most seem.to relate to easily. The Edelbrock and Rochester accomplish the same thing in a different way, power fuel enrichment. If we choose an engine of sufficient displacement to accomplish our requirements, without getting into the enrichment strategies, we will likely get better fuel economy.

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Right, and to attain the mpg goal, at some iteration in the design planning the rpm and conditions (parameters) need to be determined.
We know mixture density must increase under high load conditions, and ought to be moderately rich at idle for power.
If the engine is going to be in stop and go traffic alot, then mpg is going be greatly effected by fuel consumption at idle. (Yes a smaller engine will help greatly here)
But as you and others have pointed out, if the engine will be under load for large portions of the operating time, then it needs good part throttle efficiency and ought to be designed so to minimize the time it needs to go to high load enrichment. (A larger engine should help here, depending on gearing available and speeds that this load will typically be pulled).

If going with a larger engine but where time idling is significant, then being able to run very low idle speeds will help economy even though they require richer mixtures.

Interesting discussion.