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i wounder what are the main things for a good economy tune for big machinery or trucks. Is it really giving them more torque to reduce rpm?
There are other factors, but basically that is one aspect.
An engine uses a surprising amount of power/torque just to overcome the parasitic friction and other losses as it turns and, the faster it is turned, the greater those losses.
Another is that the engine parts are HEAVY, and their mass puts a lot of stress on the engine, parts that will live at higher rpm are expensive! The larger the parts, the greater the problem because of the square/cube law - https://www.dinosaurtheory.com/scaling.html
Higher rpm also means greater wear to the piston rings, and this could have a financial impact as 500+k miles is expected from a commercial truck engine and some long haul vehicles will easily exceed a million miles between rebuilds.
However, for economy, the most important thing may be the clean burn of the fuel, without any unburned fuel - you may have seen mention of 'de-rated' engines - these are where an engine is deliberately run at less than the designed maximum torque/power level, detuned if you like.