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Been working on this for years. Study the BSFC charts of many engines and you will notice a pattern and a fairly consistent BSFC amount for many different engines, with the largest difference between gas and diesel.
On those engines that are configured for economy, you will find that most of the performance improvements applied recently are relatively insignificant, beyond fuel delivery systems.
By this I mean at peak BSFC even a 2 valve 3.8 liter push rod Buick V6 dating back to the 60s can still compete if it has the latest fuel injection technology.
Gearing, aero, and other improvements when used in a combined strategy can have significant improvements.
With modern power plant technology the idea of a smaller engine working directly through a differential for cruising speed efficiency would probably provide some improvement to a limited degree at somewhat of a weight penalty.
My preference is capacitive storage and engine pulse and glide to the storage reservoir, with infinitely variable drives to the individual wheels and high efficiency regenerative braking for those situations where stop and go is the only option.
Engine size is not really a great factor if you consider that greater engine power would reduce the fuel consuming recharging of the capacitive storage system. In other words the current propensity in US cars to have much more engine than you really need, does not automatically mean a mileage penalty, if the fuel consuming percentage of run time is a smaller percentage of total vehicle operational time in proportion to the difference in available maximum engine power.
regards
Mech
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