Our 'hoe is only spinning something like 1500 rpm at a highway cruise. My car (honda 2.2L I4) is turning about 3000 rpm, and the girlfriend’s SUV (3.0 V6) turns something like 2500. Despite having a 5.3L V8, and essentially being a small building, it still gets like 18-19mpg on the highway. The highlander only gets 21-23mpg, despite being much smaller and weighing over half a ton less.
Big, torquey engines spinning lazily along on the highway are great for fuel efficiency. Put that into a light, reasonably aerodynamic chassis and laugh at prius drivers.
Its also the same reason my car gets the same gas mileage as an integra, despite weighing more and having a bigger engine.