According to Lyle wossname the way they work out  mpg is  start with full battery and fuel tank Run in city cycle as EV until the engine kicks in Run one more  mile cycle under charge sustaining mode measure fuel used then mpg  EV milesfuel used Im not saying thats right or even justifible I dont know where the  number really comes from and to my mind it confuses the article you wont get  once itis in charge sustaining mode That shows the danger of using the general press as references they really mangle every statement even if the interviewee is not misleading tham