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

[2TonJellyBean]

I think that perhaps the best way to answer that question is to break it down for what it is. It's a method of reducing rolling resistance. You can isolate it and measure that with a slow roll-down test. The big question, is how well you are able to take advantage of lower rolling resistance. At speed it won't make as much difference because of aero drag.

On a porker like mine, it really extends the coast and makes continual coasting on the slightest of grades possible - good thing too because it sucks more than enough gas going up these very slight grades.