every engine will have a different sweet spot, 1800 is not a best fuel milage point but rather a max power point, but this is even changing with new tech in diesel engins. this was originaly to do with power (as in electricity) direct drive generators need to run at 1800 rpm to make ac power, and there are exceptions to this now also.
you max fuel economy will be the lowest rpm you can do the work with out getting into the turbo to much. for my 5th wheel and my 14 ford 6.7 going down the highway at 95kph at just over 1400 rpm and 4 to 5 psi on the turbo gives me my best milage but my torque hits 800ftlb at 1600 rpm and stays there to 2000 rpm so new technology is taking that 1800 power spot and making it higher lower or wider as in my truck.
Steve
