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Maximum torque point being most efficient is nonsense that became widely believed because it happens to be almost true for some engines and most people don't understand how engines work. In reality, economy engines have a very limited rev range, so the maximum torque being in the 2-4k range and the maximum efficiency being in that range as well is just a convenient coincidence of there not being that wide a range of operating speeds in the first place.
You have to realize torque is a product of efficiency (thermal, the one we care about) and volumetric efficiency. Take in a lot of air and burn it inefficiently, you could still end up with more torque than taking in a lot less air but burning it more efficiently.
It's hard to tell exactly where you're going to be most efficient, but generally it's at relatively high loads and a reasonable rpm. If the engine is spinning too slowly then the heat has more time to escape, if it's spinning too fast friction is eating up too much power. As far as load goes, there's a big "fixed cost" of friction at any given rpm (by that I mean, there is a large component of friction that you can think of as being load-independent), and most gasoline engines are controlled by a throttle plate which wastes energy, so you want to be producing as much power as possible all else being equal.
However not all else is equal; modern engines are clever and can squeeze extra torque out by running rich (worse for efficiency, not all engines do this), and adjusting valve timing and such (could possibly reduce efficiency), so you don't want 100% of the available power. Also, at higher loads, more uncaptured energy leaves the exhaust. That's how you get the 75-80% number.
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