Is it more fuel efficient to cruise in top gear compared to lesser gears when cruising?

sorry wall of text.

TL:DR I agree with you, but it depends on the powerband of the engine and what type of engine.

Yeah I agree with you on the first part, I think we're basically saying the same thing, but I didn't mean dropping down a gear and accelerating FASTER, I meant more like two cars going the exact same constant speed uphill, one is going 1k rpm lugging the engine at full throttle because its out of the power band, the other is going 3.5k half throttle where it's in the power band where it's more efficient at producing power.

Lets look at this another way with cyclists:

Scenario #1 cyclist rides up a hill with constant speed in top gear with low RPM. Scenario #2 cyclist rides up a hill with constant speed in lower gear with high RPM.

Purely from an energy balance diagram, assuming the same mass and same speed, both scenarios require the exact SAME energy to move them, no more no less. But humans, much like engines, are not 100% efficient throughout the RPM range. A strong sprinter who is very efficient at high torque low RPM output(diesel or low revving V8) might be more efficient in Scenario #1, but a long distance cyclist and most normal people are more efficient pedaling at a higher RPM like in Scenario #2. So clearly either scenario can be more or less efficeint, it depends on how the engine is tuned and where the powerband is for most efficient operation.

So yeah I totally agree with your scenario analysis that accelerating slowly in a high gear is more efficient than going up hill quicker in a lower gear, but when it comes to the same speed, it really depends on where the powerband and most efficient operating RPM range is, it could go either way but yes generally lower RPM is more efficient.

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