Toyota's pioneering hybrid has seen a whole automotive segment grow up around it. With its Hybrid Synergy Drive (HSD) drive train comprising a 1.5-liter gasoline engine and 50kW electric motor, the Prius set the the standard for parallel-series hybrids. Under the revised EPA gas mileage ratings designed to more accurately reflect real-world driving, the Prius has dropped from a city rating of 60 mpg to 48 mpg. Nevertheless, it is still one of the most efficient cars on the road. It is rated as an advanced technology partial zero emissions vehicle (AT-PZEV) by the State of California.