It's downright confusing trying to calculate how much it costs to run green cars, whether it's a 50-mpg Toyota Prius or a highly efficient electric car like the 124-MPGe BMW i3.

One of the problems is how to use different efficiency ratings for various powertrains--gasoline, hybrid, electric--to calculate actual cost.

As it turns out, the AfterOilEV blog has suggested a metric that covers all of these options: How many miles do you get from different cars when you buy \$1.00 worth of energy?

DON'T MISS: One Owner's Chevy Volt Running Cost: 2 Cents Per Mile (Jan 2013)

On new-car window stickers, the U.S. Environmental Protection Agency uses miles per gallon for cars with engines, and MPGe (miles per gallon equivalent) for plug-in cars.

The MPGe measurement is the distance a car can travel under electric power on the same amount of energy that's contained in 1 gallon of gasoline.

The ratings underscore the point that electric cars are far more efficient in using energy than gasoline or diesel vehicles.

2016 Chevrolet Volt

While the 50-mpg Prius is the most efficient car sold in the U.S. without a plug, the BMW i3 comes in at 2.5 times as efficient: It's rated at 124 MPGe. No gas car can touch that.

The challenge comes in comparing the running cost of a hybrid to that of an electric car.

And it's exacerbated by the fact that a majority of U.S. residents can't answer this question: "So how much do you pay for 1 kilowatt-hour of home electricity?"

ALSO SEE: Electric-Car Efficiency: Forget MPGe, It Should Be Miles/kWh (Mar 2013)

The AfterOilEV "Mp\$" metric page lets you specify the prices of different types of energy on which a vehicle can run.

It includes not only gasoline, diesel, and electricity, but also biodiesel, compressed natural gas, pure ethanol, E85, and even E20, E30, and hydrogen.

A miles-per-gallon rating can be entered for gasoline vehicles (not diesels, only), and default prices can be changed, where you know current local pricing.

Gas pump

Gasoline, for instance, is presently under \$2.00 a gallon in many regions of North America, increasing its cost advantage over diesel fuel.

Meanwhile, the average electric rate is about 12 cents/kWh--but standard rates can vary from 3 to 25 cents per kWh.

On the other hand, energy costs have changed since the page was last updated in May 2011.

MORE: Energy Department Launches 'eGallon' To Explain Electric-Car Cost, Efficiency (Jun 2013)

Nevertheless, it's a very clever way to look easily at the distance you can drive for a dollar across different vehicles with various alternative powertrains.

HINT: Electricity wins most of the time.

[hat tip: Devin Serpa]

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