2011 825i with 6000 miles.
I always knew this thing did decent with fuel economy, despite driving in the WV hills. But after changing out the stock weights/spring for the 855D parts, to lower cruise RPMs by roughly 1000, I'm even more amazed! And it's sooooooo quiet now!
We weren't driving to conserve fuel, either(probably quite the opposite since we were testing a new mod)... just mindlessly driving in the mountains. Constant elevation changes of between 1800-3000 feet, as you can see in the attached image of my tracking app. Average speed was around 14-15 mph. I imagine on level ground, it would get 32-35 mpg. With road tires inflated correctly, maybe even 40!