Gas milage won't be any different between a non-flex fuel and a flex fuel motor on regular pump gas. The difference will be that the Flex fuel motor will have stainless fuel lines, tank, and will have the ability to sense the amount of E85 in the tank, and adjust the fuel tables accordingly.

The flex fuel motors can run on E85 which is usually alot cheaper, but you won't get any better mpg on it.

Personally I wish I had a Flex Fuel motor as E-85 is usually $.50 cheaper per gallon in my area.