[user removed]

I always thought of 80% load as based on RPM, not just the peak torque at the highest reading. Few high powered cars will spin tires at 80% load, in higher gears, at 1600RPM, probably none I have ever driven and that covers hundreds, including a De Tomaso Pantera, 390 Mustang GT, 383 Formula S Barracuda, etc.

If 80% load is a reading of about 2 inches of manifold vacuum then you can easily achieve that reading at low engine RPM. Maybe I am wrong, but the peak torque reading when on a dyno would be at precisely the same RPM that would produce the most power for the least fuel. The problem is that point is not really practical for any sustained period of time.

Within the practical operating range of most engines if you put precisely the correct (and constantly increasing as engine speed increases) load, then you can run that engine at 80% load from say 1200 to 6000 RPM by controlling the load through the dyno.

Seriously overpowered vehicles can still perform economically as long as they are not designed for such high power levels at high RPM that they suffer serious efficiency problems at low RPM. Even a NASCAR engine can get 6 MPG at 180 MPH, because that is what it is designed to do. Variable valve timing, fuel injection and feedback mixture control systems make it much more practical versus cars like the late 60s BOSS 429 Mustang that would barely idle at 1300 RPM. Today with variable valve timing and fuel injection, versus a toilet delivering fuel, the same engine could be made much more practical for everyday use.

regards
Mech