225 is likely the biggest effect, but there is something going on more than just that...probably driving habits and average speeds, as well.
In the summer, I average ~7.2-7.5 mixed with 235/255 18's. So far, I see 6.9-7.3 with the winters and similar driving habits. 235/255 > 225 is a noticeably narrower width and likely influences consumption.
The only way I ever get into the 5.3-5.6 range is by driving at or below 100km/h...not a very easy thing to do in this car.
2009 335d Sport - Space Grey
, Gloss Black
on Black Leather