Deere is claiming 18.93 hp-hr. per gal. at the pto. It has one or more heat recovery devices.
JOHN*DEERE 8320R TRACTOR SETS FUEL EFFICIENCY RECORDS AT NEBRASKA TEST
But diesel-electric as in rail or marine applications is probably more efficient over all. One challenge in automotive is running the engine at optimum load all the time. So a tiny diesel generator, storage device, and a electric drive.