I will not argue the MPG aspect because diesel has a edge because of higher compression ratio needed for it to run also yeilds higher thermodynamic efficency (auto makers and gas truck owners are hung up on
87 octane so they cannot raise CR ratio higher to boost efficency) and the fuel has a higher energy content so no surprizes here. As far as best ratio for a gasser that depends on the load and the engine and there is not blank rule but it is not a 3.55 or 3.73 like some think. ANother edge you have with diesels as shipped is that they operate at or very near their peak VE (Volumetric Efficency, which is also its torque peak) when cruising where as a modern gas motor usually peaks past 3000 RPM and some as high as 4000 RPM (not good for a tow motor). Like a oil furner a gas motor will use the least amount of fuel per HP hour produced when working hard but most are hung up with lower RPMs and efficency drops and MPG sucks.