[Christ]

By definition, an engine is struggling. It's a working piece of machinery. If it's not working, it's wasting (or off).

If we're going to have an actual discussion here, we need to straighten out a few definitions. Firstly -

Efficiency has nothing to do with fuel economy. Efficiency literally refers to the amount of energy extracted from a unit of fuel, nothing more.

An engine CANNOT be over efficient. There is no such beast, I'm afraid. Efficiency's goal is to reach 100%. Over efficiency would be considered overunity. Not possible, at least in today's science.

Fuel economy is a completely different beast from Engine efficiency. Fuel economy isn't the amount of work you make per unit of fuel. It's the distance you travel per unit of fuel.

They are only related insofar as if you increase engine efficiency, you could also have increased fuel economy for the range which has the increased efficiency.

In other words, if your engine is 20% efficient at 2,000 RPM (cruising), and you're getting 10 MPG at that engine speed, you could (theoretically) make the engine more efficient, by 50%, so you're at 30% efficiency, giving you the option to use less fuel (not 50% less) to do the same work (cruise), which would give you better fuel economy equal to the amount of fuel you are no longer using distributed over the miles you've traveled.

To clarify -

20% efficient means you're only using 20% of the fuel's heat value (BTU) for work. The rest is wasted.

Increasing from 20% to 30% efficiency means that instead of wasting 80% of your fuel's heat energy, you're only wasting 70% now. The gain is 50%, but you're only using 10% less fuel (I know, that makes it seem not worth it, but it's the way math works.)

Since you're using 10% less fuel to make the power you need to cruise at 2,000 RPM, you're also using 10% less fuel to go the distance/time that you normally go at 2,000 RPM.

If that distance/time were 100 MPH at 2,000 RPM, and you were originally getting 10MPG at 100MPH at 2,000 RPM, but you're now using 10% less fuel to go 100 MPH at 2,000 RPM, you're now capable of getting 11 MPG at 100 MPH at 2,000 RPM, an increase of 10% fuel economy.

Your theory about low-load is also scientifically inaccurate, because an engine is less efficient with less load. The BSFC chart for an engine assumes 100% load and 100% throttle. It changes with throttle angle, and it changes with load variances. The most efficient engines in the world are constantly under load, they don't operate outside of being loaded.

If your theory were true, you'd be getting better gas mileage idling in your driveway than driving down the road.