Been noticing a slow but certain decrease in fuel mileage

[Tiny Turbskie TDI]

Ever since about October i believe it was, I've been noticing my fuel mileage is a bit lower than usual. At first i just assumed i had been driving more aggressively than normal but after several months of it going from averaging 41-42 to 37-39 I'm wondering if it could be a fuel filter needing replaced (which I'm going to do soon anyway.) I also have a suspicion that the cold has been making my brakes gum up more than normal which could be a contributing factor (i also intend to check these to be sure they aren't sticking), or maybe just these cars tend to get worse mileage over winter? Any input appreciated!

 

[CableJockey]

If you live anywhere in the north where it gets cold, you are likely experiencing the result of winterized diesel. The additives to prevent gelling in cold weather decrease efficiency.
A few mpg is normal, sometimes more. When spring comes and stations begin selling summer blend fuel, you will likely see your economy return.

 

[ZippyNH]

Winter fuel has less BTU's, aka less energy per gallon, just like bio-and bio-blend Diesel fuel....
Winter fuel in some areas is a blend of #1&#2 Diesel or is chemicaly treated (there are limits).... similarly a diesel will burnore fuel just the heat up...look.at how much longer your cars stays at a slightly higher rpm alter start.

 

[9755]

These guys are right. I personally see about 5 mpg loss in the winter in my 01. The gfs 01 also sees about the same loss. My truck drops between 5-8 mpg in the winter. Just the nature of the beast.