Public Comment

The US government publishes fuel economy ratings for new cars, to inform consumers of what they can expect if they buy a particular model. For people who care about the environment, the EPA ratings can be used to estimate the global warming effects of buying and driving a particular vehicle. 

For gasoline-powered cars, the ratings are in miles per gallon, and the comparison between different vehicles is straightforward. A car that gets 20 miles per gallon does twice the environmental damage as a car that gets 40 miles per gallon. 

For electric cars, the ratings are in kilowatt-hours per hundred miles, and the comparison between different vehicles is also straighforward. A car that uses 40 kilowatt-hours per 100 miles does twice the environmental damage as a car that uses 20 kilowatt-hours per 100 miles. 

When comparing a gasoline-powered car against an electric car, it gets a little more complicated. To simplify this process, a miles-per-gallon-equivalent can be calculated for an electric car. However, there is more than one way to calculate such an equivalent. The Union of Concerned Scientists uses one method, and the EPA uses a very different method that gives very different numbers. 

THE UNION OF CONCERNED SCIENTISTS 

In their June 2012 paper "State of CHARGE Electric Vehicles’ Global Warming Emissions and Fuel-Cost Savings across the United States", Don Anair and Amine Mahmassani present "An Apples-to-Apples Comparison of EV and Gasoline Vehicle Global Warming Emissions". They define an MPGghg where ghg stands for greenhouse gases. 

They explain MPGghg by stating: "If an electric vehicle has an MPGghg value equal to the MPG of a gasoline-powered vehicle, both vehicles will emit the same amounts of global warming pollutants for every mile they travel." 

This definition makes sense, an electric car that gets 40 MPGghg does the same environmental damage as a gasoline-powered car that gets 40 MPG. 

THE EPA 

On their website and on new car window stickers, the EPA rates electic cars with "MPGe:Miles per Gallon Equivalent 1 gallon of gasoline=33.7 kWh". 

This EPA version of MPGe gives very different numbers than the MPGghg of the Concerned Scientists. For the 2015 Tesla Model S, the EPA MPGe is 89 and the Concerned Scientists MPGghg average for the United States is 45. 

WHY THE DIFFERENCE? 

The EPA number is not an apples-to-apples comparison between an electric car and a gasoline-powered car. 

The EPA compares the thermal energy in a gallon of gas, how much heat it produces when it burns, to the electric energy that comes off the power grid. These are two completely different things. 

In generating electricity, about half the thermal energy in a fuel is lost. So it is no surprise that the EPA number differs from the Concerned Scientists number by a factor of 2. 

HOW COULD THIS HAPPEN? 

Any competent scientist would agree with the Concerned Scientists, that the Tesla Model S does the same environmental damage per mile as a gasoline-powered car that gets 45 MPG. And yet the EPA gives it a rating of 89 MPGe. 

For some reason, the EPA decided to provide an apples-to-oranges comparison. 

Yes, there is as much thermal energy in a gallon of gas as the electrical energy a Tesla uses to drive 89 miles. But the laws of physics make it impossible to generate that electricity from that gallon of gas. 

I will leave it up to the reader to decide if the EPA is lying to us, but I would say their calculations are very creative. 

WHY DID THEY DO THIS? 

As an Electrical Engineer, I can see no valid scientific reason for using the wrong MPG equivalent. 

However, I can see a possible political reason: Billionaire Elon Musk, the founder of Tesla, is a major contributor to both the Democratic and Republican political parties. 

It is the duty of the EPA to give us accurate ratings, so that we can make informed decisions about our effect on the environment. But if the EPA used the correct MPG equivalent, who would buy a $70,000 Tesla that does more environmental damage than a $25,000 Toyota?