If you’ve been following the developments in the world of electric cars, then you probably recall the Great EV Claim of 2009: that the Chevy Volt would get the equivalent of 230 mpg and the Nissan Leaf would earn a 367-mpg rating.
After the story dangled out there in the media for 14 months, it disappeared and was replaced by a less spectacular version late in 2010, which said that the “new” numbers for the Leaf and Volt were 99 mpg-e and 93 mpg-e, respectively.
The new numbers are much more reasonable, of course. The Environmental Protection Agency (EPA) reached those figures by running a test cycle, determining how many kilowatt-hours were burned per mile, converting that to BTU/mile, and finally dividing that number by the BTUs in a gallon of gasoline.
But the wild difference between the new and old numbers raises questions: What method was used to calculate the old numbers? How could such ridiculous figures ever see the light of day? How could they have remained in the public eye for so long? And, most importantly, what does all this say about our ability to understand the benefits of electric cars?
Information about the old calculation procedure is sketchy. In 2009, Nissan told Design News that the 367 figure was reached by using an EPA figure of 82.048 kWh/gallon of gasoline, and dividing that by the energy burned by the Leaf (0.223 kWh per mile).
Turns out, though, there are problems with both numbers. In a 2010 EPA drive cycle assessment, the energy-per-mile figure jumped to 0.337 kWh per mile – in other words, the car burns about 50% more energy than they had previously thought. Still, that accounts for only about half the gigantic difference.
As best as we can tell, the other half of the difference lies in the use of 82 kWh/gallon of gasoline. Apparently, this number is not accepted by anyone who knows about the energy content of gasoline, yet it remained out there for 14 months before the figures were publicly changed. In its 2010 methodology, the EPA employed a figure of 33 kWh/gallon, instead of 82.
The bottom line is that the Leaf dropped from 367 mpg-e to 99 mpg-e in one quick stroke.
To be sure, the EPA never released a description of its original draft methodology, so the public will never really know how the old numbers came into being.
But the sudden drop in the miles-per-gallon equivalency figures illustrates an important point: The methodology for determining mpg-e is still slippery.
Even the Progessive Automation XPrize Foundation, which continues to be a voice of reason on this subject, has acknowledged that such calculations are difficult.
“We’re still in the learning stage,” notes John Shore, former senior advisor to the Automotive XPrize Foundation. “It all depends on the test cycle. You can take an electric vehicle, drive it at 40 mph on a flat road, without any accelerations or decelerations, and get one number. On the other hand, if you drive around corners, braking, accelerating, decelerating, you’ll get a very different number.”
Shore commends the EPA and says that its current methodology is a solid one. He warns, however, that all such ratings are still a moving target.
In other words: At this early stage, it’s best to take it all in with a dose of skepticism.