The laws limited emissions in the standard government test cycle, and the thresholds on emissions were chosen based on how well cars could be made to perform in

The laws limited emissions in the standard government test cycle, and the thresholds on emissions were chosen based on how well cars could be made to perform in those test cycles. While actual defeat devices were illegal, the legal emissions limit wasn't actually the real-world emissions limit and everyone knew it. (The EU is now gradually moving to real-world emissions testing and a more realistic test cycle, though judging from the other reply they might have some cheating problems with that - especially as real-world testing inherently has wider errors than lab testing.)