What truck, what does it weigh?
My 1 ton weighed 4,400 lb and was comfortable running about 2,400rpm for days on end. 1955 model 235, about 50-55 mph.
If your truck is lighter you can get away with lower RPM, and if going faster you generally need more RPM to offset the added wind resistance, which adds up real fast.
Each situation will be a little different and each engine might seem more content at a different RPM.
