not messed with E85 tuning too much, but unless you are buying your fuel from a race fuel supplier, at the gas station the e85 content will be all over the place. Plus when you switch do you have a easy way to drain the tank completely? Most of the guys I helped had a sensor like this which identifies the ethanol content, and a good ECM should be able to be set up to automatically adjust for fuel content, no need to do two different fuel maps.
http://www.maxxtuning.eu/products/engine...ethanol-contenthttp://www.ecmtuning.com/wiki/ethanolsupport
