Originally Posted by robrjohnson
Not really. Under-inflated tires can impact mileage because you're increasing the contact patch (more friction) but going over the recommended pressure doesn't do much to increase mileage because beyond the recommended pressure; you're not really reducing the contact patch size, just making the tire harder.
Even at 10psi lower than recommended, you might only see a 0.5 mpg drop.
For the stock tires you get a recommended pressure-I haven't seen many swapped tires that come with a recco'd psi. They have a max..but that's the only number I remember seeing on the spec sheets.
I'm all for the "You gotta try it out for yourself and find out what you like for a ride" method. After you like how the tires feel for your normal application-Make sure the pressure is still within specs for the tire.
For this one- my last comment is most important- Use the stock recommendations for a starting point and then adjust from there.