[Dazzler]

Yes, Mary is quite attractive..

Another factor with the cold and wet is increased rolling resistance with the tires as well..

[AlanHo]

The trouble is - my wife would want to accompany me.....

So now I've done one tank of brimmed diesel. Odometer said 6.0 and my caculation was 16,5 L/km. So it's pretty accurate.
I still do 130 on freeways and 90 on highways. The 90 is mostly in order to use the 6'th gear, as the engine gets noisy below 1500 rpm.. Temperatures has been below zero during the period.
After i filled it again, i did 120 km on highway in above zero degrees, and the Odo says 4.8. So temperature has indeed influence.

[Phil №❶]

I assume you did some city driving in your 1st tank. This will push up your fuel consumption too.

Driving @ 130 will use more fuel than at 80-90 ks. I know this is too slow for your roads, but I'm just talking about wind resistance on your car. 

http://eartheasy.com/move_fuel_efficient_driving.html