With recent trips to Montreal and MINIsOnTop, I have seen MPG averaging close to 30 – GBMINI loves long runs at sensible highway speeds:
GBMINI#3 MPG chart
Conversely I get my worst MPG in cold weather when the engine takes ages to warm up and I drive just the short (11 miles) distance to work.
I also expect MPG to drop 10% if the JCW upgrade gets installed (if it is ever released/approved for the auto transmission!)
Overall, my good MPGs on GBMINI#3 are within 5% of the best on GBMINI#2, before that car got the JCW upgrade.
The auto transmission clearly does not hurt fuel economy very much – driving style has a much worse effect!