Measuring the energy efficiency and environmental friendliness of different vehicles is easy if they're powered by gasoline, less so when the energy comes from grid electricity.
This leads to perpetual debates over the assumptions used on the source of electricity and its carbon footprint, among others.
But for 19 years, the American Council for an Energy-Efficient Economy has issued annual rankings of the environmental friendliness of vehicles for sale in the U.S.
Today, the ACEEE released this year's Greenest List, naming the 12 vehicles with the highest Green Scores for the 2016 model year.
For the third year in a row, the winner is the two-seat Smart ForTwo Electric Drive Coupe and Convertible, with a score of 63.
2015 Chevrolet Spark EV
The same score was achieved by the second-place finisher, the Chevrolet Spark EV, while third place went to the Fiat 500e at 62 points.
Fourth-, fifth-, and sixth-place finishers all received 61 points. They were, respectively, the new Toyota Prius Eco, the Volkswagen e-Golf, and the Nissan Leaf S and SV models.
Of the 12 greenest cars identified by the ACEEE, seven were battery-electric, one was a plug-in hybrid (the 2016 Chevrolet Volt), and four were conventional hybrids without plugs.
The council notes that for the first time ever, every single winner had some form of electrification--no cars using solely gasoline or diesel made the grade.
2016 Toyota Prius Two Eco
That was partly due to Honda ending production of its Civic Natural Gas variant, although the ACEEE notes that the gasoline versions of the Smart ForTwo and Chevy Spark "hover just below" the 12th-place finisher.
The seventh electric car was the Kia Soul EV, while the hybrid list also included the (non-Eco) Toyota Prius, the Prius C, and at number 12, the Volkswagen Jetta Hybrid.
The Greenest List is paired with what ACEEE calls the Meanest list, those 2016 vehicles that are the least environmentally friendly of any on the market.
Those 12 vehicles included seven large or high-performance luxury SUVs: five from Mercedes-Benz, one from Lexus, and one from Toyota.
2013 Mercedes-Benz G65 AMG
It also named two commercial vans (one from Chevrolet/GMC, one from Ford) and a model of the Toyota Tundra pickup truck.
Only two passenger cars made the Meanest list, both from British luxury maker Bentley: the gigantic Mulsanne and the V-12 Continental/Flying Spur lineup.
ACEEE also issued what it calls its Greener Choices list, essentially those gasoline vehicles that are most environmentally friendly--aiding the many buyers for whom many plug-in vehicles are not locally available.
That list is topped by the 2016 Toyota Prius Eco and Prius C.
2016 Toyota Prius C
For 2016, the ACEEE suspended all Greenest List rankings for diesel vehicles from Volkswagen, Audi, and Porsche due to the Volkswagen diesel-emission cheating scandal.
As for methodology, the council notes that the Green Score is largely based on life-cycle greenhouse gas and emissions of criteria pollutants (CO, HC, and NOx).
This year, it updated its calculations to take into account new estimates of emissions from fuel sourcing, transportation, and distribution, along with Clean Power Plan adjustments to emissions due to electric generation.