About Our Green Car Ratings

By jeff  |  May 13, 2010  |   blog

SUMMARY

High Gear Media bases the largest part of its Green/Mileage Rating for new cars on the vehicle’s gas mileage, as ranked by the U.S. Environmental Protection Agency. The fuel a car consumes over its lifetime makes up roughly four-fifths or more of its carbon emissions profile, so fuel efficiency plays by far the largest role in its environmental footprint.

While recycled and recyclable materials, zero-landfill manufacturing, and other such green credentials are admirable, in the end, they reduce a car’s impact on our environment far less than does the amount of fuel it burns. We will note such efforts when they stand out, but on their own, they will not drastically alter the green rating we give a car.

For a more detailed discussion, see below.

GREEN RATINGS EXPLAINED

According to M.A. Weiss et al., in their 2000 report from the MIT Energy Laboratory, On the Road in 2020: A Lifecycle Analysis of New Automotive Technologies, fully 75 percent of a vehicle’s lifetime carbon emissions come from the fuel it burns over its lifetime, with another 19 percent coming from the production of that fuel.

Extraction of the raw materials that make up the vehicle adds another 4 percent, and only 2 percent of lifetime carbon is due to the manufacturing and assembly process. While hybrids may be slightly higher in raw materials and assembly, due to their added battery pack and electric machinery, the difference in overall lifetime carbon in manufacturing hybrids and conventional cars is  negligible.

DIAGRAM: EarthTrends

Many manufacturers now promote and publicize various ‘green’ initiatives, both for specific cars and within manufacturing facilities. Ford, for example, has now rolled out renewable soy-based upholstery foam in multiple car lines. Subaru’s manufacturing facility in Lafayette, Indiana—where it makes Legacy sedans and Outback wagons—sends no waste to landfills, either eliminating or recycling manufacturing scrap, and using any leftover as fuel.

High Gear Media applauds these and many other sustainability efforts. But their actual, practical impact on carbon is minor compared to the effect of raising or lowering gas mileage. Our ‘Green/Mileage Rating’ is largely based on how well a car’s fuel efficiency stacks up against competitors in the class, those cars it is likely to be compared against by buyers.

Note, also, that a difference in gas mileage between 10 and 20 miles per gallon has far more effect on total fuel consumption than the difference between 33 and 50 mpg. The first change adds or subtracts 5 gallons over every 100 miles driven, while the gasoline consumption varies by only a single gallon for that same 100 miles in the second case. (Unfortunately, two-thirds of Americans surveyed get this exactly backward.) So we focus more heavily on gas-mileage differences for less efficient vehicles (e.g. pickup trucks, full-size cars) than we do for the higher-mileage classes (mini-cars, subcompacts, and compacts).

Vehicles running on alternative fuels are often more efficient than their gasoline counterparts. This includes hybrids, clean diesels, and the very small number of cars that burn natural gas. As for plug-ins, those whose battery packs can be recharging by plugging them into a wall socket, the EPA hasn’t yet released its methods for rating the fuel consumption of plug-in cars.

But according to a study by the Electric Power Research Institute and the Natural Resources Defense Council, any mile driven on grid power has a lower carbon footprint than burning gasoline to drive the same mile—until you hit 50 mpg or better in the gasoline powered car. Accordingly, we’ve rated any plug-in model at the top of its segment, and we look forward to the day when there are enough plug-in models that buyers will assess them based on differences in their efficiency.