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Better mileage measures for green cars: Why MPDs will beat out MPKWHGs

Car mileage, as it’s widely understood, is naturally expressed as “miles per gallon”, as in miles traveled per gallon of fuel consumed. So what happens when you begin to substitute electrons for liquid fuel? It’s a problem today’s hybrids haven’t yet had to deal with because, despite the on-board batteries, all of the energy they use starts out as gas. (The car starts moving using petrol, it only generates and stores electricity from its brakes when the car slows or when the engine works like a generator.) Things start to get really messy once you switch to plug-in hybrids since they run on a mix of gasoline from a gas station and electricity from your plug. Suddenly, it’s no longer simply miles per gallon, but miles per kilowatt hours (of electricity) plus gallons (of fuel) or MPKWHG. Doesn’t quite roll of the tongue. And few consumers would get it. An easier way is needed…

Maybe the easiest way to reconcile these apples and oranges would be to do away with energy units entirely, and use dollars instead. This would simplify the analysis and give consumers a gut level understanding of what it costs to travel a given distance. By this measure, a car that goes 30 miles on a gallon of $3.50 gas would be about "8.6 miles per dollar" car, or 8.6 mpd. Ditto, if a an all electric vehicle's battery pack can be charged with 10 kilowatt hours of electricity overnight, at about 10 cents per kilowatt hour, and it can travel 50 miles on that charge, its "mileage" would be 50 miles for about one dollar, or an impressive 50 mpd. (Both of these scenarios are possible today, although the EV would be very expensive.) For plug-in hybrids running on gas and electricity, the rating would be their driving range divided by the cost of all the gas and electricity used to make them go that far. Maybe not simple, but surely graspable. This concludes today's math lesson. But expect to see more ruminations on this topic down the road. In a recent piece in the NYT, it's evident that Toyota and the EPA are beginning to wrestle with this question too.

Toyota and the Environmental Protection Agency are mulling over how to describe the advantages of adding plug-in capability to a hybrid. The current test, which gives the Prius an E.P.A. estimated mileage of 55 m.p.g. for combined city and highway driving, does not work. Toyota estimates that for a daily commute of 15.5 miles, running costs of this prototype will be about 8 percent lower than the current Prius if the battery is charged during the day, and 41 percent cheaper if charged at off-peak rates where time-of-day electricity pricing is available.

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