The RPM's can't be the same, because say you use 8Kw of power to cruise at 60km/hr, that 8Kw of power at 14.7:1 occurs at say 1500rpm, and at 17.7:1 occurs potentially at 1700rpm. Therefore shifting your cruise rpm into a lower bsfc zone.
Remember max power for most engines occurs around 12.5:1 AFR's and anything beyond that reduces power.
I had a wideband system from way back (LM-1) that outputs a narrow-band signal so I've used that to set my Closed Loop mixtures.
