thanks guys, all good points. What I still don't get though is as far as I know - for a given mixture lets say 20°C lop in the B58 the engine, producing say 195 hp it would take the same fuel to produce 195hp at this mixture setting at sea level or at 5,000ft. If no, why not?
And if you are producing 195hp at 5,000ft, does this mean your TAS is higher because of reduced drag, or do propellers lose efficiency proportionally meaning you don't get money for nothing i.e... go faster for the same fuel.
So if the chart plotted horsepower instead of engine settings, then all the lines would slope from the southwest to the northeast - increasing altitude for the same HP increases range. Have I got it!?
Following that thought - if leaned accurately every time then fuel flow becomes your pseudo BMEP gauge, indirectly tells how much horsepower the engine is producing. Or have I gone too far.