Gas grade vs mileage
I never noticed any correlation between gas grade and mileage in my 2002 325i (std transmission). If the car automatically adapted to the cheaper gas, that factor alone didn't seem to noticeably impact mileage. What did matter was highway vs city (you can see the road trips on the attached graph) and also my gradual trend towards being less aggressive on the gas pedal. When I finally started consciously trying to optimize gas mileage (on the very last road trip), I suddenly got 32mpg instead of 29mpg.
Other than a few experimental periods, with no apparent change in mileage, that car spent its whole life on Regular/87.
Assuming that's still going to be the case, my motive to buy higher grade gas with a new car would be long term preservation of the motor against wear or crud build-up... but does it really make a difference?
Last edited by tbraithw; 09-03-2018 at 05:08 PM.