According to [1]: "Volkswagen hasn't explained exactly why it cheated, but outside analysts have a good guess. The NOx emission controls likely degraded the ca

According to [1]: "Volkswagen hasn't explained exactly why it cheated, but outside analysts have a good guess. The NOx emission controls likely degraded the cars' performance when they were switched on — the engines ran hotter, wore out more quickly, and got poorer mileage. Some experts have suggested that the emission controls may have affected the cars' torque and acceleration, making them less fun to drive. (Indeed, some individual car owners have been known to disable their cars' emission controls to boost performance, though this is against the law.)" So basically, they wanted the cars to have acceptable performance in real-world conditions, which pushes the emissions up. [1] http://www.vox.com/2015/9/21/9365667/volkswagen-clean-diesel-recall-passenger-cars http://www.vox.com/2015/9/21/9365667/volkswagen-clean-diesel...