One should get slightly better fuel consumption in the summer...
Because in the summer you do not have a very cold engine,a cold engine has higher friction and it will take longer for the oil etc to get to normal temp during cold weather !
Another factor in the winter is that you may have your engine idling for 5 mins or so while you deice your windscreen
this does affect your average mpg
In the winter my diesel was doing 52mpg (avg),it is now doing 54mpg(avg)...what
does affect diesel mpg is short journeys!
On most modern diesels even the cruising speed does not affect the mpg dramatically,a high speed long distance motorway dash will not cost me any more than 2 mpg.