Back in May of 2008 I went on a road trip with my cousin in his 08 Cooper S with a 6-speed. I got to drive the entire length of
Skyline Drive through Shenandoah National Park and then a couple hundred miles in southern Virginia. I averaged about 44mpg.
This was doing about 35 - 45 mph on moderately windy roads with some significant elevation changes. At 35 mph I had to stay in 5th gear to keep the turbo calm.
My cousin said he typically gets about 38mpg driving on the Washington beltway at rush hour. I guess that's good for his slightly aggressive driving.