As I have posted elsewhere labeling such a plug-in as getting 250 mpg is a mis-statement of fact, IMHO. A better way of stating what such a car 'gets' would be:
50 mpg + 200 extra miles on initial charge. After the plugged in battery is exhausted, thsi car has to lug around a greater mass than a non-plug-in, and thus would get reduced mileage.