Ok here's some real world data on a 35i with 7900 miles on it. Drove 167 miles averaging speed between 80-90 mph. I filled up prior to leaving and then filled up at my destination. I used 7.6 gallons 89 octane. Do the math it's 21.9 mpg. The computer said 21.7. I think it gets 1-2 mpg better with 93 oct but BMW says use 89 oct on the gas tank door. Diesel where I live is $3.99/gal where 89 oct is $3.14/gal. So in my area it's just not worth it for the diesel. Also the air temp was between 58-62 degrees.
2008 335i coupe Dinan stage II
2012 X5 35i MSport