> But how much more damaging? Not much, especially since diesel cars are a very small fraction of VW's sales in the US.

If they are really spewing 40x the limit, then we're talking emissions equivalent to almost 20 million cars that do meet the limit. This is around 1 to 2 years of total new cars sales, or roughly 10% of the active light-duty vehicle fleet in the US. Europe allows more pollution but I doubt it is 40x, and they have a lot of diesels.