Did changing the stat help with the problem? I have noticed with my A6 2.5 TDI quattro that it's doing somewhere in the region of 20mpg and that it runs very cold. The temperature barely moves unless I'm sat in traffic. I've not had it very long and had other things to fix (ABS ECU and a fuel leak!) which took priority but now looking at the fuel consumption. Given the low running temperature it sounds as if the thermostat could definitely be the culprit?