Recent experiences with high-profile battery-electric, plug-in hybrid, and extended-range electric vehicles have inspired industry-wide head-scratching about the best way to measure cars' energy use. The confusion only gets worse as gasoline and fossil-fuel diesel are joined by greater amounts of compressed natural gas, biodiesel (algae being a promising source), cellulosic ethanol, electrical energy (from various sources), perhaps widespread hydrogen (I'm not counting on it)-and who knows what else you chem majors are stewing up in your garages.

If you've followed the government's efforts to produce more instructive Environmental Protection Agency mileage stickers-spotlighted by the Nissan Leaf's and Chevrolet Volt's-you've noted the subject's vocabulary is necessarily, and rapidly, expanding well beyond "mpg." Equivalent gallons? Kilowatt-hours/100 miles? Practice saying them.

We're throwing our hat in with "kW-hrs/100 miles" as a metric to supplement our road test spec chart's standard EPA mpg information.

Although the "kilowatt-hours" part sounds biased toward electrical energy, "watt" is a nondenominational measure of power (James Watt was a steam engine guy). Multiplying it by hours turns the pairing into a measure of energy; the "kilo" bit preceding it means "a thousand of these." One kW-hrs/100 miles is how many thousands of watt-hours of energy are consumed over 100 miles. Not so bad, right?

You can convert the content of any type of energy into kilowatt-hours. A gallon of gasoline equals 33.7 kW-hrs, according to the EPA's conversion factor. A gallon of diesel becomes about 37.1 kW-hrs. By volume, diesel contains about 10-percent more energy (see the value in kilowatt-hours?).

The EPA's "equivalent gallons," proffered as an alternative to the kilowatt-hour, has an understandable folk appeal, as it pins the bigness of the bang of those other energy types to our well-established mental picture of what a gallon of gas can do. It's appealing, but intellectually dishonest. A perpetuation of our popular, dead-end thinking about energy in terms of one "chosen" unit of volume-gasoline's-just as it's beginning a decline from the world stage. Future conversations will be about energy, not liquid "volumes" of anything.

You'll note we haven't configured our new spec chart entry as distance traveled divided by energy-the form mpg takes, but the other way around. There's a good reason.

A ratio that divides what you get (miles) by what's required to get it (gallons) is called a measure of economy. Flip it around (gallons per mile), and you get something better, a unit of consumption. The reason it's better is that our old friend, the innocent-looking metric, mpg, is actually very deceptive.

For instance, say you and your green-car-loving brother down the street decide to separately take a 100-mile trip to visit Mom. Originally you were planning to drive the Bugatti Veyron, which gets a 10 combined mpg, but you decide, gosh, that's not very good, so you opt for your Porsche 911 Turbo Cabriolet instead, which delivers 19 mpg. Your gas-stingy brother has the same idea and decides to drive his 50-mpg Prius instead of his 41-mpg Honda Insight. You both have improved your trip mileage number by 9 mpg. What you won't realize until you compare gas station receipts is that your "9-mpg" improvement saved you 4.7 gallons while his netted just 0.44. Same trip, almost exactly the same improvement in mpg, but the difference in fuel saved is off by a factor of 10. That's the problem with "miles" divided by "gallons."

However, if you and your brother had measured your trips in kilowatt-hours/100 miles, your Bugatti's energy consumption would be 337 kW-hrs/100 miles, and your Porsche's, 177, for a 160 kW-hrs difference in energy used. By contrast, your brother's Insight would have used 82 kW-hrs/100 miles compared with his thriftier Prius' 67, for a savings of 15 kW-hrs. And 160 versus 15 is actually what you and your brother's choices are saving each of you. No tricks. It's immediately obvious. You can easily compare them, and no matter what, a lower kilowatt-hours/100 mile value means you're using less energy.