Could this be something that is vehicle dependent? I've read that some of the
Jeep Cherokee's out there get better mileage with higher octane gas depending on which computer it has in it.
I personally use Iso-Heet in every tank. At one point I noticed a big drop in mileage(10.9MPG at lowest). Someone suggested trying some so I did. Noticed a big change after just the first tank. I went back and forth a couple of times and watched the mileage go up and down 1-2MPG's. So I figure it's worth the $1.75 a tank.