Originally Posted by JohnMc

Lambda is the direct WBO2 sensor reading, it's the measure of how much oxygen remains in the exhaust. That is interpreted by a gauge to show an AFR value, but that interpretation involves some assumptions about the fuel, and generally speaking most gauges (unless you deliberately reprogram them) will be expecting you to have burned gasoline.

But the number, even though technically not the correct AFR for ethanol or E85, is still useful for tuning. 14.7 is still stochiometric if you're burning gas or E85. There's no real need to convert it to an actual E85 AFR.

That said, E85 acts a little differently, and using a regular setup AFR gauge or not, you might want to do some different tuning under boost. Either way, really, it's more resistant to detonation, so you might not need to run as ruch under a given level of boost. And alternatively/contradictory, it will keep adding more power a bit richer than gasoline will, so if you're really wanting max power richen it up more than you would with gas.