Auto Manifesto

January 11, 2008

The 150 MPG Fallacy

Electric cars are the way of the future. I’m convinced. But one of the biggest concerns I have is that media and proponents of electric hybrids continue to mislead and falsely state inflated fuel economy figures. For example, I saw this CNN clip today about an extreme-hybrid “car of the future”. And it’s inaccurate.

The energy required to move a vehicle is the same as before. It’s a modified Saturn VUE. So the drag and weight aren’t much different from stock. If anything it weighs more due to extra batteries and the electric motor. But for the sake of discussion it’s the same car.

That means it takes the same amount of energy to physically move it from Point A to Point B. Now instead of doing it with gasoline, you’re using electricity for much of the trip.

It should be acknowledged how much net energy is consumed during driving. There needs to be a way to equate total energy consumed for distance traveled. The reason is that a PHEV (Plug-In Hybrid Electric Vehicle) is often recharged by plugging it into an outlet, and the electricity is coming from somewhere. A 150 mpg figure is misleading for PHEVs. It makes for great headlines but it isn’t real.

It’s analogous to saying you got by last week by eating only 2 hamburgers (i.e. gasoline) and neglecting to mention the 20 helpings of spaghetti (i.e. electricity) you also ate to get through the week. If you use energy from sources additional to the gasoline in a car’s tank, it should be included in the fuel economy calculation.

Finally, it is important to recognize that the true significance here is not that the car get’s better mileage or not. It’s the fact that it offers a way to wean ourselves off of oil because electricity can come from many different sources.

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  • Internal combustion gas engines are only about 20-25% efficient - most of the energy in the gasoline is wasted as heat.
    Modern batteries and electric motors are 90% efficient.
    So if you have 10gallons of gas and get 25mpg in a 20% efficient car you can go 250miles.

    In an electric car that is 90% efficient if you could store the electric equivalent of 10gallons of gas you could go 1125miles equivalent to 112.5mpg. This rough calculation does not take into consideration further energy efficiency improvements that can easily be realized like regenerative braking.
    We don't have quite that range yet, but that type of efficiency is possible today with an electric vehicle.
    With electric you can charge your car at home which further improves efficiency because transportation costs are reduced.

    By Anonymous Anonymous, At March 11, 2008 at 1:24 PM  

  • All this is true, but irrelevant. What should be said is the total energy cost for moving the vehicle just like the author says. It is just that most of the public is unfamiliar with and put off by the appropriate ways to do so. We all have been hammered to death with "mpg, mpg, mpg" Which is fine with our current idiom. However, with the advent of PHEV's this is incredibly inaccurate. I appreciate what the xtreme people are trying to do with marketing there technology but, I would have more respect for them if they also include a more accurate representation for comparison to standard gasoline vehicles without all of the science so most people can get a better grasp of what is going on with there technology.

    By Blogger Unknown, At April 10, 2008 at 9:34 PM  

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