By John Goreham

BestRide.com EDITOR’S NOTE: This article appears in Auto Buying Guide 2017. Find the full magazine at http://gatehouse.morecontentnow.com/article/20170316/NEWS/303159999.

The first step in buying a green car is to define what “green” means in the context of personal transportation. One simple definition of a green vehicle is one that uses significantly less petroleum and generates significantly less polluting and greenhouse gas emissions than the best conventional vehicles the same size and shape.

That is no longer an easy hurdle to clear, with cars like the midsize Honda Civic earning 36 mpg combined and 42 mpg highway.

The first resource for comparing green vehicle credentials is the EPA and U.S. Department of Energy’s website, FuelEconomy.gov. One might argue that the EPA sometimes oversimplifies green vehicle specifications, but look deeper and it is easy to find quite a bit of good data to cross-shop green car choices.

Begin by selecting a vehicle you find of interest at the site and clicking on “Find a Car.” Our example above is the Toyota Prius, the longest-selling green car in the world, and still today’s runaway sales leader. Like many car models, the Prius comes in a variety of sizes and trims. We chose the most fuel-efficient gasoline-only Prius, the Prius Eco, for this example. Below we will use its plug-in hybrid variant to explain how plug-in vehicles are rated.

The red arrow we inserted points to the fuel efficiency ratings of the vehicle. Note the combined rating: That is the one you want to look most closely at. It will be the most realistic number to help you predict your mileage in normal, everyday life over a period of time. Also note the type of fuel listed, in this case, regular unleaded. That is the least-expensive liquid fuel in America and helps improve fuel economy, which is easily compared by looking at the Annual Fuel Cost number near the bottom of the chart.

The vehicle’s range on one fill-up is shown by the black arrow. Note the blue arrow at the top shows another tab we can check out for more green vehicle credentials.

The second tab of the website shows two critical metrics we can use to evaluate how green a given car is. We have switched to the Prius Prime Plug-in Hybrid for our example. The red arrow points to the annual petroleum consumption. The unit of measure is barrels of petroleum. Note that this is not barrels of gasoline, but rather the precursor, “crude” oil used to make the energy that the vehicle consumes. This metric is the one that disproves the myth that diesel vehicles are in some way “green.” It takes more crude oil to produce a gallon of diesel than it does a gallon of gasoline, negating the slight efficiency advantages some diesel vehicles offer.

The blue arrow denotes the EPA Smog rating on a scale of 1 to 10, with 10 being the cleanest. The Prius Prime scores a perfect 10. The grams per mile of CO2 produced is also shown. The Prius Prime generates 78 grams per mile. Compare that to the mainstream Honda Civic’s 248 grams per mile, and it is easy to see the green advantage a Prius Prime has over a gasoline-powered, non-hybrid car.

Battery-electric cars that use only electricity are of course the pinnacle of efficiency. Or are they? A comparison of the efficiency of the Prius Prime and Chevy Bolt above shows that the Prius Prime has an MPGe score of 133, higher than the 119 MPGe the Bolt earns. MPGe is a score that the EPA came up with to help green car shoppers accustomed to the MPG ratings of conventional cars compare the energy efficiency of various types of vehicles.

Look below the large 133 and one will see that the Prius Prime is more efficient when operated as an EV than is the Chevy Bolt. It consumes 25 kWh per 100 miles and the Bolt consumes 28 kWh per 100 miles. This electricity consumption rating can help an owner calculate cost per mile for energy.

The EPA makes it easy to get a sense of that by showing the estimated annual fuel cost at the very bottom (blue arrow). The estimate shows these two vehicles hve equal energy costs using national average electricity and gasoline prices.