Meet the greenest cars of 2013

That is the finding of the American Council for an Energy Efficient Economy (ACEEE), a US nonprofit that released its annual list of the greenest cars of the year this week.

The Toyota Prius C subcompact hatchback received the top billing, barely edging out the lease-only Honda Fit EV. How can a gas-burning hybrid be considered more “green” – already a difficult value to quantify – than an emissions-free EV? As with most things, the devil is in the arithmetic.

The ACEEE offers this very simple formula: GreenScore = a x e –EDX/c / (1+EDX/c)b

Understood? Before delving into the list, perhaps a quick summary of what that means is in order.

The council considers emissions, both smog-forming ones and greenhouse gases, produced by the vehicle , as well as by utility companies whose electricity charges the plug-in vehicles. They also examined the energy required to build the vehicle and the environmental impact of disposing of it at the end of its life, as well as fuel-economy ratings issued by the Environmental Protection Agency. Toss in a little eye of newt and presto – your winner, the Prius C, which is rated by the EPA at 53mpg in urban and 46mpg in highway driving.

Being a purely electric vehicle, the Fit EV naturally has no mpg. It has instead miles per kilowatt-hour of electricity expended. The EPA says the Fit EV travels 3.9 miles per kilowatt-hour in the city and 3.1mpkWh on the highway. A judiciously driven Fit EV can travel for roughly 80 miles on a single charge.

Next on the list is a three-way tie that includes the old green standby, the Toyota Prius; the plug-in hybrid version of the Prius, the Prius PHV; and the Honda Civic Hybrid. This illustrates how difficult it is, even with a magical formula, to decisively declare one technology or approach greener than others.

That is because while the plug-in Prius might seem cleaner than a Prius that derives its motive power from gasoline, it has manufacturing and recycling challenges associated with its bigger battery pack. Consequently, the plain-jane Prius and Civic hybrids are rated as equally green as the plug-in Prius, because that car might draw electrical power from a wall socket that is in turn fed by a grid carrying energy derived from dirty coal.

Next comes the Honda Insight, a compact hybrid that has never really caught on with buyers. For example, Volkswagen’s roomier and better-executed Jetta Hybrid finished only a point behind the Insight in the council’s 2013 survey, and costs virtually the same amount. By the council’s calculus the Jetta surrenders almost nothing in environmental impact while its driver gains a more rewarding vehicle.

Tied, however, with the Jetta Hybrid is a pair of conventional gasoline cars with no electric drive of any degree: the Smart ForTwo and the Scion iQ – marketed in Europe and Asia as the Toyota iQ. These cars trade size for futuristic propulsion.

Another point behind those cars is the Ford Focus Electric and theToyota Prius V, the wagon version of the regular Prius. With efficiency of only 3.3 mpkWh city and 2.9 highway, the Focus – as a rather luxurious and heavy car – is not as efficient as the Fit EV.

Finally, the Ford Fusion and C-Max hybrids round out the list with the biggest gasoline engines and most total power of any of the cars. Consumers and automotive journalists alike have had difficulty achieving the EPA-certified combined rating of 47mpg. Results of around 40mpg are more typical for the Fusion and C-Max, which still seems solid, considering their size.