I just switched to premium and the difference is noticeable.
The low speed stumble/bog is reduced, full throttle acceleration doesn't surge nearly as much, and on the freeway, part throttle response is improved, and the PowerShift doesn't hunt gears as often.
Keep in mind though, here in California our regular is 87, and premium only 91 (AKI), so our premium is more like mid grade elsewhere.
I haven't driven enough since switching, but I wouldn't be surprised if my MPG is improved.
Now that gas is $4.27 for regular here, the additional .20 is a smaller percentage of the total, and takes less additional MPG to cover the cost of premium.
Although I generally haven't believed in the "premium gets higher MPG" idea, I must admit that the last two tankfuls of premium have yielded about 5% higher MPG's. I was thinking we might have gone back to summer blend gas, but maybe not.
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