Hybrid systems use regenerative braking to capture kinetic energy instead of allowing that energy to escape unused as heat. They also pause engine combustion when the car is stopped, in order to save fuel.
According to Cars.com, there are two types of hybrid systems available: the kind in which the engine is dominant, as with Honda and Saturn, and the kind where the electric motor is dominant, as with Toyota, Ford, Mercury and Lexus.
Fuel economy is one of the most popular ways of measuring the efficiency of hybrid vehicles. Initially, vehicle manufacturers test pre-production prototypes, in controlled laboratory conditions, and report the results to the EPA. The EPA reviews the results and confirms 10 to 15 percent via their own tests at the National Vehicles and Fuel Emissions Laboratory.
While new EPA testing methods better account for actual city and highway driving conditions, your fuel economy may vary based on factors such as aggressive driving, maintenance habits, use of air conditioning, and weather.
According to the EPA, their estimated combined mpg is calculated by assuming 55 percent city driving and 45 percent highway driving. You can calculate combined mpg using the equation
(.55 x city mpg) + (.45 x hwy mpg) = EPA estimated combined mpg
Emissions rating is another environmental consideration when purchasing a vehicle. According to the EPA, hydrocarbons and oxides of nitrogen combine with sunlight and form smog, which can irritate the lungs and eyes. While catalytic converters, exhaust gas recirculation and electronic fuel controls reduce pollution, emission levels still vary among vehicles. A score of 0 - 10 describes a vehicle's emissions rating. A score of 10 is best and 0 is worst.
2008 Hybrid Cars Ranked by Emissions Ratings (According to the Union of Concerned Scientists)
2008 Hybrid SUVs Ranked by Emissions Ratings (According to the Union of Concerned Scientists)