Re: GT Fuel consumption
The 18s will make a difference. i have 18s on mine and generally get 500-550 for a full to no range tank. the cold will have an effect but if you're driving long, steady runs I would say not much. One thing to watch out for is diesel quality. Most diesel has some vegetable oil added, normally rapeseed. From what I have read some of the (cheaper?) brands can be of a lower grade, having a higher proportion of this. This can give variations in performance and hence fuel economy. Have you changed where you fill up?
that's my 2p anyway