How Green Is That Electric Car? And When It Hits 100 M.P.H.?

Porsche Taycan

The Tesla Model S and the Porsche Taycan give environmentally conscious speedsters an outlet for their desires.

They only look like conspicuous polluters.

A new breed of electric performance cars, including Porsche’s Taycan and the Tesla Model S P100D, shows how environmentally minded fans of horsepower might square their circles.

A supercar with a carbon footprint that seems closer to a jet engine’s than to a Prius’s may feel irresponsible in the face of climate change. But what about electric vehicles that can keep pace with or even outperform the likes of Lamborghini?

The Tesla Model S can sprint to 60 miles per hour in slightly more than two seconds, making it one of the quickest machines on the market. Is it notably cleaner than a comparably fast gasoline-fueled car like the BMW M5, which is powered by a fuel-hungry 617-horsepower twin-turbo V8?

The numbers say yes. The Tesla is convincingly the green choice, but there’s more to the story.

Even small, less powerful electric vehicles haven’t always been cleaner than the most efficient gas-powered autos. A 2012 article in The New York Times summarized a report from the Union of Concerned Scientists that found the environmental benefits of subcompact, modestly powered electric cars like the Nissan Leaf depended on where they were charged.

At the time, many states still relied heavily on coal-fired plants for electricity, and the investigators found that in some areas, electrics were no cleaner than efficient gasoline-powered cars when factoring in the emissions resulting from electricity generation.

E.V. technology has advanced considerably since then, and electricity generation in America has shifted, as well.

The latest report from the Union of Concerned Scientists, in a February article by David Reichmuth, its senior vehicles engineer, is much more optimistic than the one eight years ago. After analyzing all emissions — including those from fossil fuel production, along with conventional vehicle tailpipe emissions and power plant emissions — the group found that electric vehicles were responsible for about 10 percent less overall emissions in 2018 than they were just two years earlier. Emissions generated during vehicle and battery production or in the mining of lithium for E.V. batteries were not part of the calculation.

In this study, the average electric vehicle in the United States was found to be responsible for emission levels equivalent to those generated by a gasoline vehicle that gets 88 miles per gallon. In areas where a lot of coal is still burned to make electricity, the electric vehicle m.p.g. equivalency number can fall to as low as 49 miles to a gallon, but those areas are few and less densely populated than regions with clean power.

OK, but what about electric supercars like the Model S and Taycan? Since they produce mammoth horsepower, doesn’t it follow that their emission levels are high as well?

“A very powerful electric performance automobile is less efficient than a hyper-efficient E.V. but still far cleaner than a comparably powerful car that burns gasoline,” Mr. Reichmuth said in a telephone interview. He added that a Model S driven in California, which has some of the nation’s cleanest electrical power, is about equivalent to a gasoline vehicle that achieves 120 m.p.g. In other words, in an area with relatively clean electric plants, this extremely powerful machine can be cleaner than even the most efficient gas car.

The numbers Mr. Reichmuth cited assume that the Model S is driven responsibly. With the throttle held wide open, a Model S will gobble up the watt-hours. While Tesla doesn’t provide data for aggressive driving, some Tesla owners have explored the extremes. One estimate on Tesla’s web forums claims that at full throttle the car will use about 869 watt-hours of electricity per mile and have a range of about 88 miles on a full charge. In simple terms, that means driving 30 miles at full throttle would require about the same amount of electrical energy that an average American home uses in one day.

Driving at wide-open throttle at length would quickly heat the Tesla’s battery, triggering electronic safeguards that would slow the vehicle. So the Tesla isn’t going to take on gasoline rivals in an endurance race. But its fun-to-drive factor is very high, and in short sprints, it is nearly unbeatable. In one 2016 drag race captured on YouTube, a Model S takes on a 707-horsepower Dodge Challenger Hellcat, and emerges the victor.

The Taycan, according to Car and Driver magazine, is rated even quicker, but the magazine editors recorded identical 70 MPGe power consumption with both cars on a 300-mile trip at 75 miles an hour. (MPGe is an acronym for miles per gallon equivalent, and it’s the government’s way of quantifying the efficiency of electric vehicles. The Environmental Protection Agency, officially, pegs the Tesla at 97 MPGe combined city and highway driving, and the Porsche at 68 MPGe combined.)

The discrepancy in the Tesla and Porsche E.P.A. ratings is likely due to the structure of the test and appears to indicate that the Tesla has an efficiency advantage over the Porsche in stop-and-go city driving. No gasoline-powered high-performance car can be driven anywhere near as economically as the Tesla or Porsche electric.

A comparison of E.P.A. ratings suggests that the least economical gasoline-powered cars emit more than twice the emissions of the most economical gas car. For example, the Mitsubishi Mirage G4, with its three-cylinder engine, is E.P.A. rated at 35 m.p.g. combined, while a Ford Shelby GT 500 Mustang earns a 14 m.p.g. combined rating.

The spread between the electric extremes is much narrower. The Hyundai Ioniq Electric, one of the most efficient electric vehicles, is E.P.A. rated at 122 MPGe, yet the Tesla Model S Performance car earns a 98 MPGe rating.

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