How much of an increase in mpg can I expect with overinflated tires?

[garyhgaryh]

I wouldn't run your tires way too much over what the side wall states.
Over inflating your tires will wear out the center. If you think about it, it will also give you less contact patch to the ground, that is one reason why you are getting lower rolling resistance and getting better mileage. But you pay for it at the end with premature tire wear. Something to keep in mind. You get short term benefits of better gas mileage at the expense of your tires.
Gary