If you want a gasoline engine that is greener than a fully electric vehicle, you'll have to buy a car that's a lot more fuel efficient than the one you're probably driving now.

A new study by the University of Michigan Transportation Research Institute finds that gas-powered vehicles need to average 55.4 mpg in the United States or 51.5 mpg worldwide in order to produce fewer greenhouse gas emissions than a battery-electric vehicle. In Canada and France, the numbers would be even higher, 169.5 mpg and 524.6 mpg respectively.

The disparity depends on what is used to make the electricity that charges a battery. In countries where coal (or oil) is king, generating electricity for a full charge creates more carbon dioxide emissions than in places where hydroelectric power, for example, is the main source.

In weighing the impact, the UMTRI researchers, Michael Sivak and Brandon Schoettle, also considered the impact of extracting and transporting the raw materials for either electricity or gasoline production. The study looked at only fully electric vehicles, which are known as battery electric vehicles, -- not plug-in electric hybrids -- vs. gas-powered cars.

The Free Press obtained a draft copy of the researchers' study.

"The reasons for conducting such a country-by-country comparison are that the indirect emissions from (battery-electric vehicles) depend on the mix of fuel sources used to generate electricity and countries differ widely in their fuel-source mix," Sivak said in a news release.

Sivak and Schoettle reviewed data for 143 countries, finding wide disparities in those values. Albania, which produces all of its electricity from hydroelectric power, was at the high end of what a gas vehicle's mpg would need to be to beat a fully electric vehicle. At the other extreme were Gibraltar and Botswana, where electricity is produced from either coal or oil. The study relied on data from the Union of Concerned Scientists and the International Energy Agency.

The study did not consider the impact of manufacturing the vehicles, but did note that the Union of Concerned Scientists has found that building a mid-size fully electric vehicle results in 15% higher emissions than building a mid-size gasoline-powered vehicle. Larger battery packs push that gap to 68% higher for full-size vehicles.

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The data comes during a time of uncertainty for fuel economy standards and electric vehicle incentives. During a visit to Ypsilanti in March President Donald Trump opened the door to loosening current fuel economy standards that require automakers to achieve a Corporate Average Fuel Economy of 54.5 mpg by 2025.

In addition, the latest tax proposal from House Republicans would end the $7,500 tax credit for electric vehicles.

Contact Eric D. Lawrence: elawrence@freepress.com. Follow him on Twitter: @_ericdlawrence.