So I have recently installed the Hypertech Max Energy on my '99 5.3L Sierra. I set it up for regular fuel tuning, adjusted the tire size, and shift firmness and left everything else alone for now. I've only had one opportunity to do a real accurate MPG test, but on that one I saw a increase from 11mpg up to 13.8mpg driving at 60mph. To me this is a more than satisfactory gain, considering some people have reported no mph gains. My question is, to those of you who have the Max Energy installed, is it worth it to run the Premium tuning? That is, has anyone seen a increase in MPG from switching from the 87 to 91 octane tuning?