Update for first tank of 89 vs 87 in my blind test in my wife's Focus.
Result .... no change in mileage whatsoever. Same exact numbers.
I filled up yesterday and 89 was 22 cents more per gallon compared to 10 cents per gallon more when I started the test. I put in regular.
Sorry guys ... I don't see a difference in either performance OR mileage with higher octane gasoline. I'll try it again though when gas prices get lower and the difference between the grades is about a dime like it should be.
Maybe you gotta do it for more than one tank for the car to 'learn' about it's new higher octane fuel.