Rating cars isn't just about acceleration times—or at least it shouldn't be.

Unlike other car rating organizations, the Automotive Science Group rates cars according to their life-cycle CO2 emissions, based on a combination of their fuel economy and weight, as a proxy for the materials consumed in their production, as well as social and economic factors in their production.

This year, the Toyota Prius Prime earned the group's Best All-Around Performance title for its 133-MPGe EPA rating, its 25-mile electric range, low manufacturing footprint, and end-of-life recyclability.

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The awards are intended to guide new-car buyers, and thus focus on the first 6.5 years and 100,000 miles of a car's life—the average first-ownership cycle for a new car.

ASG notes the Prius's rapid improvement in social and environmental factors since its introduction 18 years ago. For example, every new Toyota Prius uses biodegradable upholstery and dashboard materials. ASG expects the Prius Prime to emit 181 grams of CO2 per mile, counting its manufacturing. and disposal.

Although the Pius Prime proved the greenest car for new-car buyers who want to minimize their social and carbon impact, several all-electric cars proved even greener than the Prius Prime over their full life cycles, including the Tesla Model 3, the Chevy Bolt EV, Nissan Leaf, and Ford Focus Electric.

2018 Tesla Model 3

2018 Tesla Model 3

Over its predicted lifetime, the Tesla Model 3 is expected to emit 161 grams of CO2 per mile. The Chevy Bolt EV is expected to emit 164 grams per mile, and the 2017 Nissan Leaf, 167 grams.

Since most new-car buyers don't hold onto their cars for their full life cycle, ASG rates new cars based on a normal ownership cycle of 6.5 years.

The carbon footprint of producing lithium-ion batteries is front-loaded into the sales of new cars, and offset gradually over time by lower energy consumption in the ASG study. 

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Electric cars, especially those with big batteries, take longer to make up for the production of their batteries.

"As you extend the Fuel Model beyond 6.5 years to the vehicle's full expected life cycle, the power grid continues to improve year-over-year, with lower CO2 emissions per kwh delivered, it builds further on the [electric car's] operational benefits in terms of reduced greenhouse gas emissions," said ASG spokesperson Colby Self in an email to Green Car Reports. "We include year-over-year improvements to the power grid, including line loss factors, and we see the Model 3 surging ahead of the PHEVs in later years."
"The Prius Prime by comparison, has a much smaller battery carbon footprint to offset through its mixed operation using gasoline and electricity. In real-world driving conditions, the 25 mile all-electric range is sufficient to cover a significant portion of daily travel needs (for the average person), a daily mean travel distance of 38.4 miles in the study. So the benefit of a small, efficient battery is threefold: 1) It lessens the battery's carbon footprint that is front-loaded in the early  phase of it's life-cycle, 2) It lessens the total weight of the vehicle to increase operational efficiency across its life-cycle, and 3) with a decent all-electric range, the average driver can go all-electric for most of his/her daily needs."

The rating include vehicles' performance against social standards set by the United Nations, including social performance, including respect for human rights, labor practices, environmental responsibility in manufacturing. Counting those factors alone, small, inexpensive cars scored big: the Mitsubishi Mirage (and Mirage G4), Nissan Versa, and Chevrolet Spark. These economic factors also weigh more heavily early in a car's life,

The top-scoring non-plug-in hybrid was the Toyota Prius C, at number 17, followed by the Hyundai Ioniq Blue hatchback. 

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