As of yesterday I’m getting an average of 11 miles/gallon whether if fill with expensive (Chevron) or cheap (independant) 87 octane. (When I buy gasoline at Citgo, which appears to be cheap, I always find that I am pumping more gasoline than my tank can hold so that’s not a viable comparison - Citgo is simply lying to the public about what is being delivered.) That’s been pretty much the “norm” since I bought the car last year.
There’s a bit more than 123,000 miles on the odometer and, no, my driving has been the same pattern for more than a year. I almost never use my car on the weekends because my wife’s Nissan is so much more efficient.