EPA ratings thru 07 and then afterwards
 

EPA testing thru the 2007 model year:
EPA testing procedure through 2007
Two separate fuel economy tests simulate city driving and highway driving: the city driving program consists of starting with a cold engine and making 23 stops over a period of 31 minutes for an average speed of 20 mph (32 km/h) and with a top speed of 56 mph (90 km/h); the highway program uses a warmed-up engine and makes no stops, averaging 48 mph (77 km/h) with a top speed of 60 mph (97 km/h) over a 10 mile (16 km) distance. The measurements are then adjusted downward by 10% (city) and 22% (highway) to more accurately reflect real-world results.

EPA testing starting in 2008:
As a means of reflecting real world fuel economy more accurately, the EPA adds three new tests that will combine with the current city and highway cycles to determine fuel economy of new vehicles, beginning with the 2008 model year. A high speed/quick acceleration loops lasts 10 minutes, covers 8 miles (13 km), averages 48 mph (77 km/h) and reaches a top speed of 80 mph (130 km/h). Four stops are included, and brisk acceleration maximizes at a rate of 8.46 mph (13.62 km/h) per second. The engine begins warm and air conditioning is not used. Ambient temperature varies between 68 to 86 °F (30 °C).
The air conditioning test raises ambient temperatures to 95 °F (35 °C), and the vehicle's climate control system is put to use. Lasting 9.9 minutes, the 3.6-mile (5.8 km) loop averages 22 mph (35 km/h) and maximizes at a rate of 54.8 mph (88.2 km/h). Five stops are included, idling occurs 19 percent of the time and acceleration of 5.1 mph/sec is achieved. Engine temperatures begin warm. Lastly, a cold temperature cycle uses the same parameters as the current city loop, except that ambient temperature is set to 20 °F (−7 °C).
EPA tests for fuel economy do not include electrical load tests beyond climate control which may account for some of the discrepancy between EPA and real world fuel-efficiency. A 200-W electrical load can produce a 0.4 km/L reduction in efficiency on the FTP 75 cycle test.
Link: http://en.wikipedia.org/wiki/Fuel_ec...in_automobiles

Future modifications with the PHEV arriving in the future=?

Hope this help realize the difference reflected in "posted" MPG figures by the manufacture.