I have recalculated our rpm-speed-fuel curves for the case "running against current", i.e. subtracted different figures (0-7 knots) for the current from the speed curve to get the SOG and calculated the nmpg (please remark that I have converted the data to your US units :-) based on the SOG and the fuel consumption at given rpm according to our files:
(continuous lines indicating SOG, dash dotted lines nmpg)
Starting at 3 knots head current there seems to be an optimum rpm for our boat giving the best range out of a certain amount of fuel. This optimum rpm is shifting to higher figures the higher the current flows against us. The red dash-dot-dotted line is indicating those points of maximized range.
However - as nice such diagrams are looking to engineers like me I am afraid that this particular one is of very limited practical relevance.
Up to now we have been able to avoid currents above 5 knots against us. And we never run our Lehmans continuously below 1300 rpm as it would give the best milage below 5 knots of current.
In most cases we just continued our journey against a current with the throttle setting we have had before...
best regards / med venlig hilsen