I suppose it is a 6.2 l Diesel.
The max. power is at 3600 rpm (155 hp) (see air filter housing sticker).
Hight idle with no load is between 3800 and 4000 rpm, depending on the quality of the govener on the IP. On the manufactures label of the IP the high idle no load is mentioned, if you understand to read the plate. Usually 4000 rpm.
The fuel cut down is not very accurate, so the engine might crank under load 3700 rpm with no loss of power.
The adviceable rpm is between 1800 and 2700 rpm. 1800 for best fuel efficiancy, 2700 rpm is a smooth rpm where the engine seems to fell well with acceptable fuel consumption, due to 14 years with that engine in a Chevy.
In a M715 since 7 years with 3200 rpm at 55 mph the fuel consumption for European prices is just acceptable. However, the engine needs some hundred hours to get used to it.
Wolf