Here are my results of adjusting the OBC MPG.
I had the dealer do our 2018 (G01) X3, since it's under warranty. Before adjustment, the error was -3.6% (actual MPG was 3.6% less than the OBC MPG). I had the dealer adjust the K Factor from 1000 to 970. After adjustment and three tanks of gas at the same pump, the error now is -0.2%.
My 2014 (F10) 535i took two tries. I found out that the definition of the "K Factor" is the reciprocal of what it is for the 2018 (G01) X3. Before my first adjustment, the recent error was about +2.5% (actual MPG was ~2.5% more than the OBC MPG). I adjusted the K Factor from 1000 to 1025. But, that caused the error to increase (actual MPG being ~5% more than the OBC MPG). So, for the third tank of fuel, I adjusted the K Factor from 1025 to 975. I filled up the car this morning, and the MPG error was -0.23%. Actually, part of that error was round-off of the odometer mileage. The actual trip odometer mileage was 432.6 miles, but 432 is used by the spreadsheet to calculate MPG. Using 432.6 miles, the OBC MPG error would only be -0.1%. Also, my favorite pump was shut down and I had to use another one.
The gas station I use is about six miles from my house, near Hardee's. Gas there is about $0.15/gallon cheaper than near my house. I combine a Top Tier Gas run with a top tier biscuit run to the nearby Hardee's. I leave the house about 6 a.m., so there's no traffic. I take the coastal road which has fewer traffic lights and a 35 MPH speed limit. On the X3, from the house to the gas station, I get about 36 MPG with the AC on. Today, in the 535i, the house to gas station run got me 30.4 MPG.
I blend my gas to get 91 AKI. This morning my actual fuel purchased was only 0.02 gallons off of my prediction (from the OBC MPG and the trip odometer).
Last edited by Autoputzer; 09-06-2019 at 06:38 PM.