I'm not sure if they actually cheated on the US MPG numbers. The advertised MPG US are quite a bit lower than the european ones. This could be a result of a different test cycle but also a result of not being able to cheat in the US. In Europe they did stuff like mixing diesel fuel into the oil supply and increasing the tire pressure beyond the allowed range. This increases MPG but will cause the engine to break early and would be dangerous on real roads.