Cold weather fuel consumption

[caldirun]

Now the weather has warmed my fuel consumption has returned to normal (1.9 tdi AVB), I have noticed that when the outside temp was showing 5C or less the fuel consumption would increase by 15%. This extra fuel consumption was not just for starting and warming-up the engine as it would continue throughout a long trip as well, until the temp read above 5C then the consumption would drop back to normal.
Over the last 2 tanks I have been getting 51+ mpg but on the cold days during the winter I was struggling to get 43 mpg. Can anyone explain what causes the extra fuel consumption at 5C or below, is this normal or is there a fault I need to address.
Would it be possible or advisable to put a switched resistor in the outside temp sensing circuit (or the one that feeds the ECU with air temp data) to switch the reading from 5C or below to just over 5 when the engine had warmed-up?

[martin1810]

The outside temperature sensor is for information only...not used by ecu. Ecu mainly adjusts fueling using info from fuel temp sensor and coolant temp sensor (not same as temp guage, but same sensor unit). I suppose you could trick the ecu if you take lots of measurements and then have a couple of switch resistors for cold weather.

[v6tdi]

This must be the norm, my 2.5 tdi passat my wifes golf tdi and a mate of mine from work also has a golf tdi and we have had this very discussion. On all 3 motors fuel consumption during the winter is a lot poorer the two cars we own have both been remapped some years ago so it cant be down to the ecu program.