McLaren P1Enlarge Photo
We usually think of automotive energy efficiency in terms of miles per gallon, but that isn't adequate for cars with plugs.
For battery-electric cars and plug-in hybrids, the EPA has created a surrogate measure for how many miles a car powered by a battery can go on the amount of energy that's contained in 1 gallon of gasoline.
That's called MPGe, or "Miles Per Gallon Equivalent."
DON'T MISS: 2016 Toyota Prius: Most Fuel-Efficient Car Without A Plug, Ever (Dec 2015)
The ability to drive for appreciable distances on electric power alone usually gives plug-in hybrids an efficiency advantage over internal-combustion vehicles without plugs.
But six plug-in hybrids are actually less energy-efficient than the 2016 Toyota Prius Eco—rated at 56 mpg combined—according to figures published on the EPA's Fueleconomy.gov website.
The dirty half-dozen emphasize performance or luxury over outright efficiency, but at least they introduce their owners to the pleasures of electric driving.
As one advocate said, "plug-in hybrids are the gateway drug for battery-electric vehicles."
McLaren P1Enlarge Photo
2015 McLaren P1 (18 MPGe)
The McLaren P1 is among the top tier of supercars, but efficiency is not part of its mission.
Its powertrain consists of a 3.8-liter twin-turbocharged V-8 and seven-speed dual-clutch transmission, working with an electric motor to produce a combined 903 horsepower and 663 pound-feet of torque.
ALSO SEE: McLaren P1 Hybrid Supercar: Production Version Revealed (Feb 2013)
The electric motor is used primarily to augment the gasoline V-8, not power the car on its own.
While McLaren has said the P1 can drive on electric power for short distances, the EPA does not rate it as having any electric-only range.
First shown as a concept at the 2012 Paris Motor Show, P1 production was limited to 375 units, with the final car completed in December 2015.
2016 Mercedes-Benz GLE550e
2016 Mercedes-Benz GLE550eEnlarge Photo
2016 Mercedes-Benz GLE550e (43 MPGe)
The GLE is a renamed version of the SUV formerly known as the Mercedes-Benz ML-Class.
This first-ever plug-in hybrid variant uses a 3.0-liter twin-turbocharged V-6 and seven-speed automatic transmission, with an electric motor sandwiched in between.
MORE: 2016 Mercedes-Benz GLE550e Plug-In Hybrid: Quick Drive (Jul 2015)
Total system output is 436 hp and 479 lb-ft of torque.
An 8.8-kilowatt-hour lithium-ion battery pack provides enough power for 10 miles of electric running, according to the EPA.
But as a big, heavy SUV, the GLE550e is more about less-inefficient luxury than truly efficient driving.