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It isn't absolutely necessary, but if you have a tune for 91/93 pump and a tune for E85, you will need to run the tank to almost empty for either to work alone without a flex fuel kit. Flex fuel kit just makes it more convenient to add E85/91/93 at any time without the tank being completely empty since it adjusts the fuel trims/maps automatically.
If you plan on running E85 full time, you can get a tune just for that and be ok if you don't plan on running 91/93 pump gas. But if you plan on switching between E85 and pump gas, the tank will need to be next to empty before switching maps.
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