With hybrid and electric vehicles appearing in more and more automobile showrooms around the world, the traditional fuel efficiency measure of miles per gallon (MPG) alone just doesn’t cut it anymore. With cars able to be powered by electric power alone or a combination of electric and gasoline, new measures are needed to better inform consumers when buying a new car. To this end, General Motors (GM) and the Environmental Protection Agency (EPA) have teamed up to design a new window label for the Chevrolet Volt that has more information than any EPA label before it.

To make the fuel efficiency of alternative fuel vehicles easy to compare to traditional gasoline-powered cars, the MPGe measurement has been widely adopted. This stands for miles per gallon equivalent and is determined by measuring the electricity use (kilowatt-hours) of the vehicle and converting it based on the energy content in a gallon of gasoline. The EPA calculates the MPGe based on a conversion factor of 33.7 kW-hr of electricity being equivalent to the amount of energy in a gallon of gasoline.

When driven in electric mode, the new label lists the Volt’s fuel efficiency as 93 MPGe, while the combustion engine provides a fuel efficiency of 37 MPG. Both figures were determined by the EPA using a combination of city and highway driving. A combined composite figure for overall fuel efficiency of 60 MPG is also displayed. The 93 MPGe figure is much lower than the 230 MPG GM was promising a year ago but the company says this is because that figure was reached using a draft proposal for rating EVs that the EPA later rejected.

The label also displays a range of 35 miles in electric mode for the Volt, (GM claims a range of 25 to 50 miles – depending on conditions), and a total range of 379 miles possible when switching to the combustion engine once the batteries are flat. There’s also how the vehicle compares to other cars in its class in terms of overall MPG, greenhouse gas emissions and other air pollutants, as well as charging time and estimations of how much it will cost to run the vehicle in a number of different driving patterns.

By way of comparison, the all-electric Nissan LEAF received a rating of 99 MPGe, while the long time fuel economy leader, the Toyota 2011 model Prius, received an overall rating of 51 MPG.