What cars available in dealers today make the most efficient use of energy?
The answer turns out to be four battery-electric vehicles and one plug-in hybrid.
Gasoline and diesel cars don't even come close; internal combustion engines waste two-thirds to three-quarters of the energy in their fuel on heat and noise.
To build this list, we simply used the "fuel economy" measurements issued by the U.S. Environmental Protection Agency.
For electric cars, the EPA measures their efficiency in what's called MPGe, or Miles Per Gallon Equivalent.
It's a measure of how far the car can travel electrically on battery energy equivalent to that contained in 1 gallon of gasoline.
So here are the five most energy-efficient vehicles offered in the U.S. today. (Only the Nissan Leaf is available nationwide.) Drumroll, please!
2013 Honda Fit EV drive event, Pasadena, CA, June 2012
(1) 2013 Honda Fit EV: 118 MPGe
Not only is the Honda Fit EV a well-executed electric conversion that's fun to drive, it ranks as the single most-efficient electric car you can drive out of a dealership today.
You can't buy it, mind you--it's lease-only.
Unfortunately for eager shoppers, Honda will make no more than the planned 1,100 Fit EVs it needs to meet California laws.
The EPA rates the Fit EV's range at 82 miles.
2013 Fiat 500e electric car, Los Angeles drive event, April 2013
(2) 2013 Fiat 500e: 116 MPGe
The electric Fiat 500e is another superb conversion of an already fun-to-drive car. It may be the best-handling electric car south of the Tesla Model S.
At the media launch, we almost got the sense that Chrysler's engineers were stunned at how good their car was.
The electric 500, like the Nissan Leaf, can be leased for $199 a month--and the first year's production is reportedly already sold out.
The EPA says the Fiat 500e will go 87 miles on a full battery charge.
2013 Nissan Leaf, Nashville area test drive, April 2013
(3) 2013 Nissan Leaf: 115 MPGe
The Nissan Leaf, now in its third year on the market, is by far the volume leader, having sold several times more than the entire rest of the list combined.
It's also built in the U.S. using lithium-ion battery cells fabricated right next door to the assembly plant in Smyrna, Tennessee.
With a base model priced at $28,800 before incentives, and a 6.6-kilowatt charger cutting a full battery charge to about 4 hours, the latest Leaf is now nipping at the heels of the Chevrolet Volt for the title of best-selling plug-in on the market.
The EPA rated the 2013 Leaf's range at 75 miles, although that calculation isn't directly comparable to the 73 miles of the 2011-2012 Leaf. It's complicated.
2014 Honda Accord Plug-In Hybrid
(4) 2014 Honda Accord Plug-In Hybrid: 115 MPGe
The only mid-size sedan among the group, the plug-in hybrid Accord is a new entry for Honda and actually preceded the 2014 Accord Hybrid that will be launched later this fall.
This is a plug-in adaptation that uses Honda's new two-motor hybrid system, with powerful electric motors and multiple modes of operation.
As of last month, Honda had sold only about 250 plug-in Accords, and only in California. It's rolling the car out slowly and carefully.
The EPA rates the Accord Plug-In Hybrid's electric range at 13 miles, during which its efficiency rating is 115 MPGe.
After the battery is depleted, it behaves like a conventional hybrid and is rated at 46 mpg combined.
2012 Mitsubishi i, City Island, NY, Aug 2012
(5) 2012 Mitsubishi i-MiEV: 112 MPGe
Finally, the last car on our list is the smallest--yes, even smaller than the Fiat 500e--and the least powerful (its electric motor is rated at just 47 kilowatts, or 63 horsepower).
The tiny i-MiEV five-door hatchback (also known for awhile as the Mitsubishi 'i) makes for an excellent city car, assuming you have a charging station available.
But its limited range and lack of power mitigate against much highway use; the EPA rates the i-MiEV's electric range at just 62 miles.
NOTE: The 2013 Scion iQ-EV is actually rated by the EPA at 121 MPGe, which would put it at the top of our list.
Only 100 of that car will ever be built, however, and it's not available for the public to buy--so we eliminated it from our list.