For my job I analyse vehicle MPGs, all diesel vehs, temperature makes a big difference to fuel consumption, watch your MPG go up as the temperature rises when Spring finally makes an appearance.
I recently found this interesting VW article
http://en.volkswagen.com/content/me...itions/rendition.file/effizient_unterwegs.pdf
Every journey is different. For a 10-km trip, a difference of 20ºC in outdoor temperatures
(roughly the difference between summer and winter temperatures) can make a 20% difference
to your fuel consumption. And if you normally travel not 10 km but just 2 km, that can increase
your personal fuel consumption by as much as 50%.