This is a question about engines and I am seeking advice from the collective wisdom of the group. Which gasoline is the best choice and why? Here is my situation. I have a 2012 FF with 24,000 miles. In my area by law ALL the gasoline must have ethanol. Everything I have seen has 10%. The choices are 91, 89, 87 octane. The manual says my car is designed "to use regular gas with a rating of 87. Using gas below 87 is not recommended. Premium fuel will provide improved performance." When I use premium I perceive my car having more power and gaining at least 2 mpg improvement on the in dash calculator. When I use 89 I still observe some increase in power and I save a few pennies. However when I use that octane when I go up an incline under light acceleration I hear a slight engine grumble. Is this knocking? The manual says "do not be concerned if your engine sometimes knocks lightly. However if it knocks heavily under most driving conditions while using the fuel with the recommended octane rating see your authorized dealer to prevent any engine damage. On this board people have said you can use premium because the computer brain advances the spark properly. I have also read manuals for the 2013 and 2014 do not include the sentence "Premium fuel will improve performance". If this is true, why the change? Should I just stick with 87 octane? Or should I use 91 octane and skip the in-between rating of 89 because it is confusing the computer brain.