My little 2.4 starts making power above 2500 rpm , 3000 is pretty good. For power on hills I often wind it up over 4000 and on some sandy gravely climbs I had it a 6000 to get me to the top. That little motor likes to rev.
I would say anywhere between 2500 to 3000 is good, for better milage around 2500 seems to work well. That's where I used to get the best milage on the highway