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.
Interestingly, this is what happens in the pharmaceutical world. The FDA has a "bad ad" program where you can report pharmaceutical advertising that falls outside regulations. Who is responsible for the majority of reports? Competing company's sales reps who see competitor materials when they visit doctor's offices.
“New cars are so much cleaner than your old car" is a cross-brand claim that is important for the whole industry. Any damage to that claim would increase replacement cycles.
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.
The day Ford execs testified in favor of bailing out both GM and Chrysler, it was clear that they have little interest in beating their supposed competitors.