I always wondered why companies don't try to catch each other cheating. If would seem that if Ford wanted to, they could easily figure out VW was cheating, and go to the press and government with that info.
The best case scenario for telling on competitors would be a larger fraction of a smaller market, worse outcomes would involve a similar or smaller fraction of an even smaller market (after symmetric response accusations hitting your own brand). You don't sell Marlboros by pointing out that smoking Lucky Strike might cause cancer.