if you increased the rolling diameter of the tire you will lose mpg. also the tire can change mpg all by its self without even changing the size. I work with tires and have seen it a million times. a hybrid will come in getting 40= mpg and not want to pay for the tires designed for eco cars and want a cheaper tire. well that cheaper tire doesnt have the same rolling resistance and weights several pounds more than the eco tire. i personally switched my stock tires (continental) for a bridgestone turanza with serenity plus which is a low rolling resistance eco tire. i average 41.8 mpg on the hwy now.
Insert smartass comment here!