Oh - OP question...
Lean ratios basically burn less fuel for the same engine speed... often lowering engine power output as well, meaning higher throttle angles, meaning less VE losses.
It's a two-prong gain - Less pumping losses will net you better efficiency, while leaner ratios means you're burning less fuel to go just as fast. (provided you're still making enough power to maintain your speed.)
Honda and several other MFG's have designed GDI engines that will run at a/f ratios of ~60:1... that said, I don't think they're produced en masse as yet, for several "great" reasons, I'm sure.
*GDI - Gasoline Direct Injection... think diesel, but with spark.
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