High load is well documented to improve efficiency. That’s a major reason why taller gearing helps, too.
Consider you have two engines, a 3.0L and a 6.0L. The 6L can produce X horsepower at 2000rpm with 50% load, while the 3.0L can produce the same amount of horsepower at 2000rpm at 100% load. Which do you suppose will deliver better economy?
The answer is almost always the 3L engine. Half as many cylinders, fewer bearings, far less friction, but more importantly it also is wasting no energy at all creating vacuum.
You could gear the 6L to run at 1000rpm, of course. That way it would also be at 100% load, making the same horsepower, just by spinning half as fast. Or in other words, a 3L engine is often very equivalent to a 6L engine spinning half as fast. Smaller engines are like taller gearing.
What happens though, when you can’t slow the engine down anymore? When load is low, but you can’t cruise at 500rpm or whatever is necessary to get load up? For this reason, there is an optimal engine size and gear set for every vehicle.
Don’t take my word for it though. Read up on base specific fuel consumption. Here’s a sample chart showing engine efficiency vs rpm vs load.
