I run one-third 100 with the rest "91" (in quotes because our gas is so bad here in Phoenix with the anti-smog additives) and my Xfire, with all the mod's, runs MUCH better, much smoother all the way to redline. Haven't actually checked the mileage, but I get more miles per tank for sure.....of course, 100 is $7.25 at the Scottsdale station here that carries it........