it's a funny one as I have a c 250D 4matic averaging 27-30mpg around town with the 20mph in central London, but 50mph on the motorway drives to Nottingham and back, but I have had the car in neutral and was able to push it easy and it rolled with little effort on a level road. Had a mechanic notice it didn't roll as others do and after cleaning the adblue and Nox and taking it for a long drive tested the thermal response of the wheels and the rear nearside was way higher than all of the others. Took to to another mechanic who said it was the discs probably warped and the pads needed changing.