We have 2 cars, a 03 Saturn Vue and a 07 Chevy Equinox. Same car. The Vue has the 4 cyl (gm) and the Equinox has the V6 (honda)The Equinox gets better fuel mileage. The tach readings are differnt for say 60 mph. The 4cyl is turning more rpm's than the v6. Part of it I would guess would be the gearing, but the V6 doesn't have to work as hard. This has been the feeling on the Saturn Forum and it looks like they were right. Both are automatic's. Well the Vue has a rubber band transmission,
"Hummmmmmmmmmmmm", no shift. "It sucks"
The point is like mytmouz was saying. Most cars-trucks are designed to have the engine running at a rpm where the powerband is at. Too low and the engine doesn't make horsepower, too high and all it does it eat fuel. Look at some dyno engine runs. You will see where their is low hp and torque and where there is more. You don't need or want max of those for gas mileage.
Oh and the flatter the curve, the better it is for driving.
There is a old saying.
"Torque gets you going and horse power keeps you going." or something like that. Not quite true, but it git the idea.
HTH.