[HummerJim]

I've done my last three fillups(each 25 gallons or more) with 89 octane on my '08 and have found it to be better at idle and under WOT than the 87 octane and not much difference from using 91 octane. Mileage has been almost identical to the 91 octane, averaging around 12.0 with more than average highway driving.