The best RPM for fuel economy is 0rpm the best for FEL will vary greatly depending on what you are doing.
No matter what I never run anything under 1500rpm that's where I will start, and as soon as it lugs or starts to slow down a lot I`ll bump it.
For plowing I normally use around 1900-2100.
Moving dirt 2000-2500 depending how much, and what's in it.
I would say most of the time I run 1900-2300 and go above if I have a heavy item.