[comptiger5000]

I put slightly larger tires on my Jeep and corrected the speedo for them. Upshifts happen at a slightly higher road speed (same rpm), and it doesn't downshift any more often (too much low end torque for the current gearing). It seems to have had minimal to no effect on mpg though. If anything, it went up by less than 0.5 mpg on the highway.

Whether or not taller gears will be useful with an auto depends on the vehicle, as well as your driving style. In town, they probably won't help. You may gain a little on the highway, if it doesn't make it downshift a lot.