At 133 MPGe, Toyota Prius Prime wallops BMW i3 on energy efficiency

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2017 Toyota Prius Prime Premium

2017 Toyota Prius Prime Premium

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Move over, BMW: there's a new efficiency champion in town this year—and you'll have to give back that laurel wreath you've worn since 2014.

Last week, final EPA fuel-efficiency results were released for the 2017 Toyota Prius Prime plug-in hybrid.

When running in all-electric mode, that car is now the most energy-efficient vehicle you can buy in the United States.

DON'T MISS: 2017 Toyota Prius Prime: first drive of new plug-in hybrid

When Toyota released the early details on the Prius Prime back in March, it estimated an electric range of 22 miles.

That turned into 25 miles by the time our first drive rolled around last month.

Now, the company's estimate of 124 MPGe in electric mode, made last month, has risen to a final EPA rating of 133 MPGe.

2014 BMW i3 (German-market version), Amsterdam, Oct 2013

2014 BMW i3 (German-market version), Amsterdam, Oct 2013

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The 124-MPGe estimate exactly equaled the rating for the most efficient version of the BMW i3 electric car, fitted with a 22-kilowatt-hour battery pack.

That car has been the undisputed efficiency leader in the U.S. since its launch as a 2014 model.

ALSO READ: What Is MPGe Anyway? What Is Efficiency For Electric Cars?

(Miles Per Gallon Equivalent, or MPGe, is a measure of how far a car can travel electrically on the same amount of energy as contained in 1 gallon of gasoline.)

A newer 2017 version of the BMW i3 electric car, with a larger 33-kwh battery, has a slightly lower efficiency rating of 118 MPGe but an electric range of 114 miles, up from 81 miles.

2017 Toyota Prius Prime Premium

2017 Toyota Prius Prime Premium

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Just like fuel-efficiency ratings for gasoline power, the MPGe efficiency rating depends greatly on the mix of test cycles used by the EPA.

While non-hybrid gasoline cars are usually more efficient at highway speeds, electric cars virtually always have lower efficiency ratings on the EPA's highway cycle versus the city cycle.

CHECK OUT: How Does The 2011 Chevy Volt Get 230 MPG? By Making Assumptions (Aug 2009)

That's because the much greater efficiency of electric drive means aerodynamic drag weighs far more heavily on electric cars than on gasoline cars whose engines continue to run even when they're stopped.

General Motors' mysterious 230 logo

General Motors' mysterious 230 logo

The importance of test cycles was highlighted back in August 2009, before the EPA had finalized the methods it would use to rate the efficiency of electric cars.

Chevrolet came under much criticism for marketing its not-yet-released 2011 Chevy Volt range-extended electric car as a "230-mpg" vehicle.

The company had come up with that figure, for what later came to be called MPGe, using a preliminary proposal for a blend of test cycles and calculations that the EPA didn't end up adopting.

Regardless, the Toyota Prius Prime is now decisively more efficient for its electric range of up to 25 miles than is the BMW i3 battery-electric version for its up to 81-mile range.

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