How can you be slower when producing 65% at best economy than when producing 65% at best power
There is a bit of a misunderstanding of physics here. The TSO-540 I referred to earlier produces 350 horse power at rated power. 65% of that is 227.5 horse power. At 227.5 horse power the aircraft will only fly at one given fixed speed (all other things being equal and not talking back side of the power curve). Whether that 227.5 horse power is produced at a best power or best economy mixture setting is completely inmaterial. Horse power is horse power and a fixed horse power produces only one fixed speed. Where the confusion arises is that for a fixed throttle setting as you lean from full rich the power output initially increases to a maximum (RPM increases with a fixed pitch prop) and the point at which the RPM peaks is the best power mixture setting. The RPM then falls away as we lean because power is being lost until we get to the best economy mixture setting. If we plotted horse power against mixture we would come up with a representative chart as follows. For a given throttle setting lets assume that best power mixture equals 100% power. If the mixture is made full rich the power output will drop to 95% and at best economy the power out put will drop to 93%. (Figures taken from the TIO-540 manual).