I know the thread is old but, just wondering about this also, isn't most tunes really only affect WOT fuel and timing, and all cruising fuel economy is controlled by the O2 sensors, like for example if you add global fuel in the tuner won't the computer just pull it out in the long term fuel trim?? And about the premium, with a 20 gallon fillup it is only 4 dollars more per tank, wow that breaks the bank!, and I did the math If you got 15mpg with premium($3.20) it would cost you 21.3 cents/mile and if you got 14mpg with 87octane(3.00) it would cost you 21.4cents/mile. As far as gas mileage goes it all about timing, I saw a small increase I added 2 degrees global timing and removed 2 degrees from all the wot tables. But I am just asking if I am right about how the computer pulls fuel with the O2 sensors.