Updating my old thread since I've kept this experiment going...mainly out of curiosity of gas price fluctuation as well as seeing if I can see a noticeable drop in efficiency when it comes time for a carbon scrub. I seem to have lost a few receipts before I could update my spreadsheet so there are a couple of gaps. This version also does not make note of snow vs 'rest of year' tires or when I had my roof rack on/off. Yes, I have gotten lazy.
You can see where I took a road trip this past Memorial Day, though. Mostly highway w/ a little city driving on both ends of the trip. #'s are down some weeks due to heavy traffic running errands on hot days. For instance, that last fill up had me at 21.5mpg. Prior to that day, I was at 24 for almost a whole week.
Still.....the OBC reading still typically tracks to within a few tenths of a gallon.