I think if you were at 55mph for the whole gas tank, yea you'd see an increase - but the problem lies in the start and stop traffic. It takes more gas/power to turn the larger tires for acceleration and I think this would negate if not make worse your gas mileage

With that said, I get the best gas milage in my camaro with it right in it's power band on the interstate cruising at about 3grand. Anything lower and I find myself stepping on the gas pedal a little more to get the same results for going up hills/etc and it generally results in lower gas milage. I think with larger tires you may end up lowering your cruising RPM enough to make the same sort of difference, IMO.