Maximum fuel economy is achieved by operating your engine at the RPM where the BSFC, Brake Specific Fuel Consumption, is logged at its lowest. The engine manufacturer has this data. Also, in response to some who said that operating at RPM below torque peak damages the engine; this is only true if the engine will not acclerate smoothly if you step on the pedal. When you take off from a stop, you are operating at less than torque peak RPM and you also know that the engine acclerates without straining. Where operating at below peak torque (lugging) occurs, the engine will not accelerate under load....this is damaging specifically to the connecting rod bearing (upper half). Your auto trans should be set up to prevent this from occuring by downshifting accordingly.
Gary Spires
Retired Cummins Field Service Engineer