Honda also designed the Civic Hybrid to meet California's tough Partial Zero Emissions Vehicle standard, so the car puts out a reduced amount of air pollution.
Lastly, the model's 110-horsepower gas/electric propulsion system and continuously variable automatic transmission team up to produce a great 44 mpg/city and 47 mpg/highway.
Unfortunately, buying all of that green technology will take a fair amount of "green" out of your wallet, as a Civic Hybrid costs some $6,000 more than what comparable gas-powered Civic sedans run.
Fourth-greenest 2014: Toyota Prius
Base price: $25,010
The Toyota Prius has been the bestselling hybrid by far since coming to the U.S. market in the 2001 model year.
The 2014 version gets kudos from the ACEEE for complying with California's strict PZEV emissions standard and offering outstanding fuel efficiency.
The EPA rates the compact hatchback's 134 horsepower gas/electric system and continuously variable automatic transmission at 51 mpg/city and 48 mpg/highway.
But like the Civic, going green with a Prius requires plenty of greenbacks. A base 2014 Prius, which Toyota considers in between a gas-powered Corolla and a conventional Camry in size, costs more than both -- some $7,400 above the Corolla and $2,000 higher than the Camry.
Third-greenest 2014 car: Nissan Leaf
Base price: $29,830 (not counting tax credits)
The all-electric Leaf offers an astounding 126 mpge/city and 101 mpge/highway in fuel efficiency and doesn't produce any tailpipe emissions at all.
"There are no on-road emissions with the Leaf, so the only things we considered were the 'upstream' emissions [from electric generation used to recharge the car] and emissions created from manufacturing and disposal," Vaidyanathan says.
The Leaf's 107-horsepower propulsion system can go an estimated 84 miles between recharges, which take around 12 to 20 hours using a standard 120-volt electric socket or about four to seven hours with a special 240-volt system.
And while the Leaf costs nearly double what a comparably sized Nissan Versa Note gas-powered automatic lists for, the costlier vehicle does qualify for up to $7,500 in federal tax credits. Some states offer additional tax credits as well.
Second-greenest 2014 car: Toyota Prius C
Base price: $19,080
The Prius C gets an "A" when it comes to environmental friendliness.
The newest, smallest and least-expensive version of Toyota's popular Prius hybrid, the Prius C lists for nearly $6,000 less than what a standard Prius costs.¿¿But the Prius C has several eco-friendly advantages over its pricier cousin.
For openers, the Prius C's 53 mpg/city fuel efficiency beats the regular Prius' rating by 2 mpg. The Prius C also has a lower test weight (2,750 pounds vs. 3,500 pounds for the larger model).
Vaidyanathan says those advantages more than offset the Prius C's environmental drawbacks, which include 46 mpg/highway fuel efficiency -- 2 mpg lower than what the standard Prius offers.
The less-expensive model also only meets California's Super-Ultra-Low Efficiency Vehicle II emissions standard, which is less strict than the PZEV specification the regular Prius complies with.
The Prius C's base price also runs some $3,600 above that of its closest gas-powered cousin, the $15,455 Toyota Yaris four-door automatic.
Rolled out for the 2012 model year, the Prius C features a 90-horsepower gas/electric hybrid system and a fuel-efficient continuously variable automatic transmission.
Greenest 2014 car: Smart Fortwo Electric
Base price: $25,750 (not including tax credits)
The 2014 Smart Fortwo Electric car has earned the highest ACEEE score ever, slightly beating out the 2013 Toyota Prius C.
Vaidyanathan says the two-seat electric car scores so well because of great fuel efficiency -- 122 mpge/city and 93 mpge/highway -- and a super-low test weight. The Fortwo Electric weighs just 2,250 pounds, second only to the gas-powered Smart Fortwo as 2014's lightest model.
The electric- and gas-powered Smart cars are also the U.S. market's smallest vehicles (just 8.83 feet long), making them both nimble to drive and park.
"It's a tiny car," Vaidyanathan says. "Whether that makes sense for [a given car buyer] is a valid question, but it's probably good for urban driving."
On the downside, the Fortwo Electric's 47-horsepower propulsion system has a 78 mph top speed and just a 68-mile estimated range between charges when used for a mix of city and highway driving.
The electric model's sticker price -- $25,750 for a coupe and $28,750 for a convertible -- might also give you a shock. Fortunately, both versions qualify for up to $7,500 federal tax credits, as well as state tax credits in some jurisdictions.
By Jerry Kronenberg