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You start running E70, or anything over 15 percent, and you have bigger problems than a fuel tank. Those vehicles that are approved for E85 have not only different fuel tanks, they have different lines, cylinder heads and all parts, pistons, etc., are hardened against the corrosive affects of ethanol, as well as the higher heat produced.
So I would recommend you burn what is approved for the vehicle; if not, you will cause engine damage that will not be under warranty.