After 4 year of driving two different w123 300TDs (85 and 84) and keeping track of every ounce of fuel, I can conclusively say that my cars get better mileage in the summer (24-25 mpg) than in the winter (20.5-22 mpg).

Whether that is because of external temps or fuel quality is up for debate.

City vs highway seems to have less to do with fuel economy for me.