i filled with a tank of 91 at BP. Truck is running with a gearhead tune. Got into some hills at 70 mph and the knock sensors were blinking red on the hills, with cruise set. Dropped back a couple gears to make things work. Mileage went from 18 to 14. If I would not have had Ngauge connected I would not of known. Burnt out half a tank and filled with some 93 and things were sort of back to normal. Not sure what crap I got into. So it looks like running an Ngauge is a good idea to know what is going on.