Well, efficiency is relative. Here are two heavy hitters.

My father has a 99' K2500 4x4 Suburban. Gen VI 454 BBC. It's rated at 290hp @ 4,000 RPM & 410 lb/ft @ 3,200 RPM. Vehicle weight - 5,600#. 4.10 gears with 31" tires. 4l80e transmission (4 speed).

I have my 15' Ram Power Wagon. 6.4/392 Hemi. It's rated at 410hp @ 5,600 RPM & 429 lb/ft @ 4,000 RPM. Vehicle weight - 7,000#. 4.10 gears with 35" tires. 66rfe transmission (6 speed).

BOTH of these trucks get 10mpg around town. Both get nearly identical fuel economy on the interstate. One weighs 1,400# more than the other, and makes 120 more HP and 19 lb/ft of TQ. Cylinder deactivation rarely if ever occurs, as I leave my truck in tow-haul mode. This shows how efficient newer drive lines truly are, even a watered down 392 Hemi with eagle heads (not Apache) is relatively efficient, for the most part, due to having 6-gears versus 4-gears.