In general:
*the higher the compression ratio the better mileage, up to about 22 to 1 NA, some mills get this same effect with a compressor. Present gasoline engines probably can't take full advantage of ethanol at 10to1, better at 12or14 to 1
*the fewer number of cylinders the better, less fiction
*the more BTU's per gallon the better, diesel at approx. 130K vs. LPG at 91K
*the Fuel Injected engine usually is better, closer control over air/fuel ratio
*the smaller engine gets better MPG, if its running near its max. torque peak.
thus if the job requires 50 Lb-ft of torque @ 2500 RPM and a certain 50 cu. in. engine's max. torque is 50 Lb-ft @ 2500, it would get better mileage than a 100 cu. engine operating at 50 Lb-ft.
*the longer the stroke, the more torque per cu.in., the lower (RPM) the torque peak, the less fiction, computer controlled engines have offset much of this advantage, because they can be lugged down without as much chance of mechanical damage. The old 2 cylinder tractors held the fuel economy records (for tractors) until recently.
To sum: the smallest, slowest speed, turbo'd, diesel, 3cyl, that can just make enough torque to propel the load on level ground will get the best MPG.
What about hills you say, that's the FORCE made transmissions.