B.s.f.c. Bs.....?

Pishta, you have the relationship inverted. At 21 mpg, the BSFC would be 1/3 that at WOT (not 3x). The only way to use 100 HP steadily at 60 mph (without accelerating) is to drive out of the LA Basin on I-5 or I-15 where you climb continuously to 4500 ft. We did that last Sunday in our 2002 T&C 3.8L (close to a slant) which has an instant mileage indicator. I recall ~8 mpg vs the normal 23 mpg on the flatlands.

In both cases, the fuel controller keeps the air/fuel ratio almost the same. What is different is that when cruising on the flatlands the throttle plate restricts the inlet air, dropping manifold pressure. The pistons still draw in about the same volumetric flow, but the mass flow is 1/3 due to the lower pressure. When climbing, with WOT, the manifold pressure gets close to atmospheric pressure.

A diesel is quite different. It always runs WOT (no throttle plate). You inject more fuel to increase horsepower. Diesels don't rely on spark ignition. The high compression (~20:1) heats the air enough to ignite each fuel droplet, regardless of how many droplets are there.