The temp gauge in these cars is also for entertainment to some extent.
The gauge will stay at 12 o'clock over a 50F range!!!!
You need to access the Hidden OBC menu, 3rd link in my signature, and display the coolant temp on your OBC display. Normal temp is 90-95C, anything lower in town or at idle is a bad stat.
DISA not working also impacts mileage a bit was well, first link in my signature. VANOS seals leaking can also impact performance and mileage as well.
But the first thing to do it check coolant temp, then change gas station and brand. What EVERYONE needs to realize is the issue if more than just the fuel mixture, it is also a function of how well the pump is calibrated as well. You NEED to pay attention to the state calibration stickers on the pumps. If the date is more than 1 year old, I would be checking for a station with newer calibration stamps.
Winter blend fuels are also and Ethanol percentages are also a factor. FYI there is a push to consider going to 15% Ethanol, not sure it has been approved yet, but Ethanol blend percentage is not closely regulated and you would be surprised what the percentage you actually end up with may be!!