and consider even higher static compression ratio that is compensated for by reduced 'dynamic' compression ratio.
The greater amount of expansion on the power stroke will help fuel economy. Compression ratio is also expansion ratio. Expanding 12 to 1 on the power stroke helps, even if you don't have the octane rating to do that on the compression stroke without detonation. Prius, Honda Insight, and Ford Escape engines all use these ideas to run 87 octane gasoline on 12 static compression ratio.
Calculate your average 'piston speed' with the short stroke 273 ci and then set up the highway cruise gearing to get piston speed around 1000 feet per minute and vacuum inside the intake manifold about 6-8 inches of vacuum. This is the 'island' of maximum fuel economy that stands above the 'ocean' of otherwise inferior fuel economy for most gasoline engines, when measured by pounds of fuel consumed to make a given horsepower for an hour. The 800 to 1200 foot per minute piston speed range is where friction & torque are optimum for fuel economy...above this speed range torque peaks, then volumetric efficiency peaks, then horsepower peaks but neither are best for fuel consumption.
see this complicated link for island and ocean fuel economy graphs of a TDI diesel converted to sparkplugs and running on methanol and ethanol:
The EPA guys above use 'mean effective pressure' and rpm, but you can use the stroke of the TDI crankshaft to convert their graph to piston speed along the bottom.
There is also a 'generalized' gasoline engine fuel economy graph in this 'classic' book by the father and son Taylors:
there is also a graph showing optimum quench area by piston diameter
Keep in mind that MPG is not dominated by engine efficiency. Most engines are nearly the same efficiency at their best spot..varying mostly on compression ratio.
To get better MPG make tire rolling resistance lower make aerodynamic drag lower make mass lower ...then re-gear drivetrain to optimum horsepower for all those improvements:
A long read on MPG improvement by tire rolling resis, aero and engine eff...almost a small book:
