European regulations dictate that cars are tested at between 20C and 30C, temperatures we seldom see in Britain. en.wikipedia.org/wiki/New_European_Driving_Cycle
The regulations are a nonsense, which in theory specify that diesel cars should emit only trace amounts of deadly pollutants (which kill tens of thousands a year in Britain), but in reality allow manufacturers to emit many times higher levels of deadly pollutants in the real world.
The latest news is that apparently diesel cars are even more noxious and carcinogenic below the frigid depths of 18 Celsius (yes, +18, not -18C)
www.bbc.com/news/business-36589106
AIBU to wonder why we are being gassed and poisoned by these noxious vehicles on the basis of completely spurious test results which correspond in no measure to what is actually emitted on the road.