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  • 230 mpg?

    Posted by David Foster on August 11th, 2009 (All posts by )

    General Motors has announced that the Chevy Volt will get 230 mpg for city driving, and probably around 100 mpg for combined city/highway driving.

    The Volt obtains this performance, of course, through its use of a battery recharged from the grid. “230 mpg” means “230 miles per gallon of gasoline,” and ignores the coal or natural gas which in most cases will supply the recharging. The Electricity Fairy has not been coming around a lot lately.

    A proper metric for a vehicle such as the Volt depends on what factors the buyer really cares about…

    If your main concern is “energy independence,” then “miles per gallon of gasoline” is probably a reasonable criterion.

    If your main concern is operating cost, then you need “total cost per mile,” based on a combination of gasoline cost and electricity cost.

    If you worry that the world is going to run out of energy, you should be looking at “BTUs per mile.”

    And if you really believe CO2 is going to destroy us all, then the metric you should care about is “CO2 emissions per mile.”

    According to this, there are people in EPA who want to calculate the mileage by assuming that the battery is recharged from the car’s internal combusion engine at the end of a driving cycle. That idea seems really preposterous, given that the buyers of this car should be those who will normally be able to “refill” the battery on a regular basis and minimize the ICE use.

    It’s probably only to be expected that GM would hype the mileage of this car, but one would hope that journalists would point out the limitations of the “mpg” methodology when a vehicle like the Volt is involved.


    9 Responses to “230 mpg?”

    1. Whitehall Says:

      But it it is all-electric, than it doesn’t have any place for “gallons” of anything?

      I think there was an engineering units conversion problem somewhere in the PR/Marketing departments.

    2. David Foster Says:

      Whitehall…the Volt isn’t all-electric…it’s a rechargeable hybrid. Run on battery till it runs out, then the engine kicks in.

    3. Isegoria Says:

      If your main concern is operating cost and thus total cost per mile, then, as a practical matter, you don’t really need to sum the gasoline cost and the electricity cost, because the electricity cost is negligible.

      It’s more eye-opening to realize that the familiar miles-per-gallon metric is upside-down. You really want your gallons per mile — or per 100 miles (or whatever) — to approach zero. That’s the number that’s proportional to your gas expenses.

      When you look at it that way, you realize that going from 20 miles per gallon to 230 isn’t much better than going from 20 to 50 — because going from burning 500 gallons per year to 43 isn’t much better than going from 500 to 200. Even at $4 per gallon, that’s just $600 per year difference between 230 mpg and 50 mpg.

    4. Paul Milenkovic Says:

      Since this Web site is about free markets, what is the problem about the particular mix of electric and gas power of the Volt — provided there are customers willing to purchase a Volt?

      I know, the problem is about a variety of market interferences. People want to interfere in the market and do this interference by non-market methods as well (cap-and-trade vs gas tax, etc), and this new kind of car comes along, and they don’t know how to regulate it or categorize it.

      So GM is touting 230 MPG, but what does that mean when some of the power comes from the wall socket? Who cares? Let consumers sort this out, let them talk to their friends about whether a Volt makes sense or not for their kind of driving. But the Federal Government cares because the Federal Government regulates what you can call the gas mileage of a car.

      Oh, and since GM is Government Owned, there may be all manners of non-market incentives to put resources into the Volt.

    5. david foster Says:

      Isegoria…entirely right that the MPG measurement is upside-down. This would be an interesting discussion point for a school math class.

      Re the relative costs of the gasoline and the electricity, my quick back-of-the-envelope analysis gives electricity (at $.10/kwh) about a 3 or 4:1 advantage over gasoline (at $3.00/gallon)…the electricity costs won’t have an overwhelming effect on the total, but aren’t completely trivial. (There are of course very significant regional differences in electricity prices.) The Democrats’ war on energy, of course, makes it very probable that both gasoline and electricity will become much more expensive–hard to say at this point which of these commodities they will hurt the most.

    6. Isegoria Says:

      [Duplicate comment deleted by request of commenter. Jonathan]

    7. Isegoria Says:

      [Duplicate comment deleted by request of commenter. Jonathan]

    8. Isegoria Says:

      [One more time. Sigh.]

      GM’s press release provides some numbers:

      Applying EPA’s methodology, GM expects the Volt to consume as little as 25 kilowatt hours per 100 miles in city driving. At the U.S. average cost of electricity (approximately 11 cents per kWh), a typical Volt driver would pay about $2.75 for electricity to travel 100 miles, or less than 3 cents per mile.

      It looks like I was quite off. If the Volt is using 0.43 gallons of gas to travel 100 miles, then that’s $1.29 for gas (@$3/gallon) and twice as much — $2.75 — for electricity (@$0.11/kWh). That sums to ~$4 per 100 miles, versus $15 for an ordinary 20-mpg car — or $6 for a 50-mpg Prius.

    9. Isegoria Says:

      To tie together Paul’s concerns about regulation and David’s idea of a math-class discussion, let’s look at how fleet mpg averages relate to actual gas consumption — because the relationship is not linear, and the incentives are all wrong.

      A uniform fleet of 10-mpg clunkers has an average mpg of 10 and consumes 10 gallons per 100 miles. If we replace half the fleet with 230-mpg Volts, the average mpg increases to 120 mpg, but gas consumption only drops to 5.2 gallons per 100 miles. So, increasing average mpg by a factor of 12 increases fuel-efficiency by a factor of 5.

      If you were GM and needed to increase your fleet’s average fuel efficiency, which would you do: increase the whole fleet’s mpg by 10 percent, at great expense, or sell a few hyper-efficient cars to boost the average mpg value?