Just for anyone who is curious, here is what mileage I get in my 88 535is with 245k miles. Pure city 15 mpg. Pure highway at 70mph, 27 mpg. It has the JC chip and I use premium.
Last night we made a run from Sacramento to Lake Tahoe. Started out at 300 feet elevation and the summit is over 7300 feet. We went for dinner and ran between 65 and 80 mph. The average for last night was 26.7 mpg.