Drive Your Car for Free!?

We need MPK - a new way to measure the energy effiency of cars.

For many years, we've used MPG (miles per gallon; apologies to the the parts of the world that use a sane measurement system...) to measure fuel efficiency of cars. But as the automotive industry converts from fossil fuels to electric cars, we will have confusion - until the government establishes a new method.

In the news last week, GM claimed that the Chevy Volt will get 230 MPG. That leads me to picture a conversation at a car dealer:

Car Salesperson: "...and the Volt gets 230 MPG!"
Customer: "Wow! It doesn't use much gas, does it?"
Sales: "That's right, it doesn't. And for the first 40 miles, it only uses the battery. Do you drive more than 40 miles most days?"
Customer: "No, just 20 miles to work and back. So usually I won't use any gas?"
Sales: "That's right."
Customer: "So... basically I drive it for free?"
(The salesperson knows that's not entirely true, and doesn't want to lie to the customer, so he just smiles.)
Customer: "And if I drive it more than 40 miles, I'll be getting 230 MPG... that's ten times better than the car I drive now! It will cost less than one-tenth what I pay now!"

Of course the problem is that electricity isn't free, and the rules allow a car company to publish a number which has little bearing on real life. As the article mentions, the Volt's battery must be recharged regularly. The GM CEO said that should cost "about 40 cents at off-peak electricity rates in Detroit." GM is not lying to the public, nor are they pretending that the only expense of operation is the cost of gasoline. Evidently the number 230 was derived by following the rules.

Unfortunately, most of us don't live in Detroit. For the USA, the average residential rate is 11.6 cents per kilowatt-hour (kWh), according to the Energy Information Administration.

What does it really cost to drive the Volt? According to the article, GM says it estimates 10 kWh to charge the Volt. That means For the first 40 miles per day, it costs $1.16. The article also says the Volt "might get as much as 50 mpg" when running on the gasoline engine. That's much better than almost every other car on the road, but not 230 MPG. If you drive 80 miles in a day, you will use a full battery charge plus at least 0.8 gallons. Together, that will cost:

  • $1.16 for the first 40 miles
  • $2.00 for the next 40 miles (average gas price of $2.50 from

Total: at least $3.16.

My car gets about 26 MPG. Driving 80 miles costs me $7.69 - more than double. (The quoted "as much as 50 MPG" means I pay less than double, but we don't really know how much more I pay than I would with a Volt.) Most of us would save money by driving the Volt.

The most important benefit of the Volt is reducing fossil fuel emissions. (I won't get into the fossil fuel emissions at the electric generation plant, 50% of which comes from burning coal...) But the misperception that the Volt - at 230 MPG - is ten times less costly than your car may become widespread, unless GM is required to report energy efficiency in some other way.

Don't get me wrong: I am happy that GM has developed the Volt, and I know that they didn't just make up the number 230. We do need to pay more attention to gas mileage and fossil fuel emissions. I hope that the Volt is successful.

And I hope that soon there will be other electric cars. But how will we compare their energy efficiency? A competing car company may state that their electric car is more efficient than the Volt. How will we know?

For electric cars, we need a new metric: miles per kilowatt-hour (MPK). That will allow us to compare the cost of driving two different electric cars.

And for mixed-energy cars (like the Volt) we will need to compare the efficiency of the gas engine (MPG), and the electric engine (MPK), and we need to know how far the car will go on its electric engine. The last one could be MPC: miles per charge. Without all of those numbers, we can't compare the costs of driving two different cars.

Sound complicated? You bet, especially when you include the need for highway vs. city driving. But until we have more information, as consumers, we'll be confused, perhaps misled, by claims like "230 MPG."


It sounds like a real good idea but GM had electric cars and decided "we" didn't want them! Now, its all the rage!
I still ask, "How long will this fad last?"

Posted by RJohnson on August 18, 2009 at 05:13 AM EDT #

The supply of gasoline, diesel and natural gas is finite, and there isn't a reasonable, naturally occurring substitute. So either future cars have batteries, or we use electricity in a factory to make a gasoline-replacement fuel (hydrogen or other).

So the 'fad' of moving away from gasoline-powered autos is here to stay.

Posted by Jeffrey Victor on October 31, 2009 at 02:34 AM EDT #

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Jeff Victor writes this blog to help you understand Oracle's Solaris and virtualization technologies.

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