Butt dynos often tell you you have achieved your goal when you haven't, they live up to your expectations, a placebo effect. Focusing on fuel to increase power or mileage is hit or miss and a home run is not much different than a base hit especially if considering the cost difference of fuel grades. Accurate measurements of the time taken or fuel used from point A to point B can be close but not easy because of all the variables unassociated with the fuel change. To remove variables dyno runs tell the true story. IMO the most cost effective way to increase mileage is is still driving with a raw egg under your foot until that becomes your muscle memory. And on the highway drive at the speed the engine is most efficient. Unfortunately, traffic will probably run you over. Rather than trying to find more benefits from fuel changes, bite the bullet and get a tune to maximize your goal.

https://www.hemmings.com/stories/201...r-priced-fuels. https://www.youtube.com/watch?v=S11g9yqSJ94

From the Hemmings article: "In summary, most modern vehicles do not require higher octane fuels, unless specifically expressed in your owner's manual (read carefully, because there is a difference between higher octane being "recommended" and "required" in the manual). "

From the Owners Manual: "REQUIRED Fuel: Premium Unleaded gasoline. Minimum Octane: 91"

BMW requires 91 Octane, they don't suggest it. Maybe there's a reason.