I did try the Techron fuel injector cleaner stuff recommended to me by another Mini owner (6 bottles for $17.99 at Costco).

Added it in the first of three tanks of high test (93) used on a recent trip, 95% highway/5% city driving, kept the cruise control (I have an automatic 2004 Mini Cooper) at about 70 for highway driving and averaged 35.5 mpg! First tank was 35 mpg; second tank was 36 mpg.

Best I've gotten in a long time, so maybe that Techron stuff DOES work, and I definitely seem to do better on high test (and the car feels peppier as well).

 

We took our first long drive in warm weather this weekend. For the first 220 mile leg going south down the Sacramento Valley, almost all freeway, temperatures were 71-81 F, and we had auto AC on 74. Winds were light from the NW, and we set cruise at 65 true mph (67 indicated). MPG average was 33.8 as indicated on our Scangauge; this was slightly downhill with 700' elevation loss.

We followed with the usual horrendous SF bay area traffic driving (mpg not noted), and then drove the return highway leg of 261 miles, with somewhat warmer temperatures (78-91), and gusty variable winds mostly from the SE. MPG average was 34.6 for 700' elevation gain, same AC and cruise setting.

 

We recently went on a trip to Traverse city-Mackinac Island from Chicago and We averaged 30.5 mpg on a 1000 mile trip. other than that I use the car to drive back and forth to work every day and my mpg now is 33.7 on the computer.
This is the non S countryman