Apologies to anyone who has read this question in
rec.autos.makers.watercooled but I thought I might get a better
response from UK VW owners.

I've just returned from a holiday skiing in France. During the
outgoing journey (in very cold conditions) I noticed the car was
returning much lower mpg than in the UK (typically about 10 mpg lower).
On the journey back I monitored the mpg and found that when the
ambient temperature was at or below 0 degrees Celsius the mpg would be
around 40-45 but when the temperature rose by just 2 degrees the mpg
would increase to 50-55. This was confirmed by the mpg meter. The
lowest mpg figures were returned when travelling through a cold flat
section of Northern France which is at sea level. The average was
taken over hundreds of miles not a short distance where the car could
be going up or down a gradient. We travelled at a constant speed
(approx 80mph). We bought fuel from various service stations but the
variations in mpg were not related to where be bought the fuel. We
travelled non-stop; the car remained hot for the entire journey and did
not go through warm-up and cool-down cycles. Once we got back to the UK
(a blistering 7 degrees), the economy has been excellent and I'm still
using the same diesel that was in the car when it was running
inefficiently.

I can fully appreciate that cars use more fuel in winter due to heat
loss, viscosity of the oil, friction of bearings, differential, gearbox
etc but I cannot believe that such an insignificant change in
temperature would have such a dramatic change in mpg.

Has anyone noticed this? Has my car got a fault with the ECU or should
it inject more fuel when the ambient temperature drops below 0?