Cool thread, a few points (given that I work with car manufacturers, do loads of miles and test various bits'n'pieces).
1) Diesel fuel formula changes through the year to take into consideration ambient temperature shifts / fuel thickness. So consumption will vary relative to fuel changes / time of year.
2) Cold air is more O2 dense, hence better combustion especially if there is moisture in the air but equally colder = more friction through cold (expanded) components and thicker oil.
3) Cruise control hurts fuel consumption compared with a gentle right foot as it maintains speed up hill. Its better to gain speed downhill then loose speed uphill.
4) The old school "use the engine for braking" is pointless nowadays, it was designed for a time when brakes couldnt take a lot of heat and were not very efficient. Nowadays brakes can take it and using them rather than the engine saves engine wear (compared with brake wear which is cheaper) uses less fuel and is better for emissions.
5) I wouldnt recommend people doing this as it may be considered dangerous and you may get Mr.Plod having a word with you. But the best run I did with 103mpg average (A2 1.4 TDi 70) was mainly motorway and I free wheeled downhill and towed lorries uphill (got in their slip stream). Best fuel efficiency trip I have ever done was Donington Park to Silverstone (65 miles) doing 49mpg in an Alfa 147 2.0 petrol at average motorway speed of 80mph - there happened to be a large van doing the same trip (quickly) that wouldnt get out of my way
- never did get chance to thank him.
6) Modern climate control systems are amazingly fuel efficient compared with their older relations. I remember a Lancia Delta integrale where you could actually see the fuel gauge move when driving with aircon on! A2 is effected but the climate control but only by a few % in my opinion.
7) My A2 has the fuel heater in it and I think that impacts consumption during cold weather more than anything. Although its useful for heating up "thick supermarket fuel that you buy off the shelves"