If I get tuned on e70 in late winter early spring how lean will it be on e85 in summer? Will it matter that much? I know e70 makes it a tad but richer going from e85, so I assume it will be leaner. Thanks
You best bet is just to get a tune for both. Your tuner should be able to dial in on e70 then adjust for e85. Come summer, just watch your STFTs, when they start going above a few percent after a fillup then switch to your e85 tune.
This. When I switched to e85 I dialed it all in on that then made another tune for e75 which I've found a few times, and all I needed to do to make a second tune was switch the stoich value and it was almost spot on.