Column: On the Road

What's in a number? Dissecting GM's 230 mpg rating
Rob Cleveland

Rob Cleveland

One thing is certain in the automobile business. When you start peeling back the onion on the performance claims and quality ratings made by and for all of the automobile manufacturers, you better have a bottle of ibuprofen at the ready. General Motors’ recent announcement that the Chevrolet Volt will get 230 miles per gallon (mpg) certainly is no exception.

First, some disclosure for your reference. I have been a big fan of GM’s efforts to create its hybrid electric vehicle and felt for some time that the car could be a positive step forward for transportation in general. I’ve been a strong supporter of GM’s many efforts in alternative transportation, from hydrogen to ethanol, as well. There are plenty of detractors around these programs – and some of the criticism is warranted. But for a company so maligned by the public as being out of date, I’ve always felt GM got a bad rap seeing how they are doing at least as much as any other car company to find solutions that take petroleum out of the transportation equation.

With that said, I found myself scratching my head trying to absorb this latest news out from GM. In principle at least, it seemed as if the Environmental Protection Agency (EPA) had just given the Volt a 230 mpg fuel economy rating, more than 10 times the average fuel economy of other cars on the road. Fantastic or fantastical?

One can be excused for having pitched one’s eyes in that too familiar arc with an element of pessimism upon hearing this. In the midst of a political, fiscal and public relations nightmare, GM manages to pull this arrow out of its R&D quiver, hitting a bulls-eye with the news media. Let the conspiracy theories begin: the EPA, the federal government agency charged with developing rules for fuel ratings, just created the rule that calculates the Volt fuel economy at 230 mpg – more than 10 times the average vehicle on the road today. And GM’s principle owner, at 60%, is … the federal government.

Or just maybe we’re witness to something genuinely revolutionary – an opportunity for those who have been railing about automobiles for years to climb off of their green dais built of cork and mud into – gulp – a Chevrolet. Perhaps after all of the chatter and hype, the American public is finally getting what it has been clamoring for, and GM is really capable of changing with the times after all.

Being in a university town, hopefully more than a few folks here will wonder what exactly is behind GM’s lofty number. Well, that is where it get’s fun. My breakdown is below but I’m hopeful that people with more alphabet soup after their last name will weigh in on the comment lines.

Get Out Your Calculators

Here is some background to provide the proper context for the subsequent calculations. The EPA is chartered with coming up with the formulas and testing procedures that dictate how car companies will calculate fuel ratings to their cars. This isn’t just an exercise. Automakers can incur heavy fines if they can’t demonstrate that their fleets meet the Corporate Average Fuel Economy (CAFE) benchmarks mandated by the government.

For the Volt, and other electric vehicles, the EPA calculates the rating, in part, based on how many kilowatt hours (kWh) are used to go 100 miles. GM says the Volt will use about 25 kWh.

So with that, let’s do some math. To do this I offer you no less than four calculations and ratings.

For the first number we have my math, which I would call the “simple” formula, seeing that it took me 10 minutes to come up with it.

If it costs about 11 cents on the average for a kilowatt-hour (the national average), to go 100 miles using 25 kWh would cost approximately $2.75. Compare that to a car getting 20 mpg – about the average for cars and light-duty trucks on the road right now. Using a national average of $2.60 per gallon of gasoline (visit to see your region), the same mileage costs $13 on our benchmark “pretend” gasoline vehicle. So the straight math tells us that the gasoline equivalent is about 4.7 times as much, putting our “simple” calculation for the Volt at about 95 miles per gallon. Not bad, but not anywhere close to 230 mpg. Yes, this calculation is imperfect, as it does not take into account whether or not the car ran all-electric or in both electric then gasoline modes. And of course as the price of gasoline fluctuates and the benchmark vehicle changes, so does the answer. But it is a simple start.

Of course, the EPA doesn’t like things to be that simple, otherwise I could just run the whole place myself with a laptop and a Twitter feed. Instead, they have conducted studies on the driving patterns of about 80,000 people and, based on those driving patterns, concluded that the average Joe (or plumber or six-pack drinker) is driving in a particular way. These “driving cycles” are used in coming up with the mileage.

The EPA takes these driving cycles – there are different ones for different scenarios like city and highway, by the way – and calculates the fuel efficiency of the Volt over a cycle, assuming one starts out with a full charge on the battery pack.

A Little More Complicated

Now we explore the second number, 230 mpg. The cycle is roughly 50 miles, and the fully charged Volt goes 40 miles on its initial battery charge, and then consumes a fifth of a gallon of gasoline to go the remaining 10 miles.

The very simple math – no more complicated than the scenario above – says that if the car made it 50 miles on one-fifth of a gallon … guess what? The car will go 250 miles per one gallon (250 mpg). The test cycle itself isn’t quite 50 miles, and there are some inefficiencies one needs to build into any powertrain system versus the theoretical world, so in the end, 230 mpg was probably a safe bet and one GM was willing to stand by, assuming everyone is inclined to take this approach at face value.

But hang on – what about all of the “fuel” that goes into that first charge of the battery pack? Surely we can’t ignore that. Seeing that electricity isn’t free at the moment, and has its own inherent energy capacity, what is the mileage relative to gasoline (if you’re not a fan of math or making life more complex than it needs to be, skip ahead six or seven paragraphs.)

And so we have the third number.

Gasoline has an energy density of 114,000 BTU/gallon. A kilowatt hour has 3,400 BTUs/kWh, so it takes 33.59 kWh to equal one gallon of gasoline (which, by the way, is why gasoline has been so popular).

This puts the conversion factor at 33,590 from watt hours to gasoline. Unfortunately, the EPA gets 33,705 so I will use their number instead, since I am using the calculator on my Mac.

Now take the fuel economy in terms of watt hours to go 1 mile (remember GM says 25 kWh to go 100 miles, so 250 watt hours to go 1 mile) and divide that into the conversion factor: 33,705/250(wh/mile) = 134.8 mpg. So in electric mode, the Volt is getting 135 mpg – much closer to my “simple” calculation result.

Of course, now this completely ignores the gas mileage that ensues after 40 miles when the onboard gasoline-driven recharger kicks in to top off the batteries. It does this when the battery is at 30% capacity and tries to keep things above that line until you can recharge.

Here GM provides us with our fourth and final number. When the engine is on, the Chevy Volt is getting 62 mpg – about 12 mpg better than a Prius, but nowhere close to the jaw-dropping 230 mpg previously stated.

What to Believe?

Deciding which answer to believe is complicated in itself. After all, it is the EPA’s own rating formula that provided the 230 mpg and GM is unlikely to make such a dramatic claim without some way to use the EPA as a backstop for the media’s fastball pitches. On the other hand, it doesn’t stand to reason that the car can get 135 mpg in electric mode, 62 mpg in gasoline/electric mode and somehow walk away with 230 mpg.

My only thought is that the EPA’s complete model for fuel economy calculations is so complex and so outdated that trying to assign a fuel economy rating in miles per gallon to an electric vehicle is like trying to publish a web page with a Heidelberg four-color offset press.

This is likely to get even more complicated as the EPA considers rules that will provide ratings for electric-only modes and composite modes (sort of the city/highway equivalent). And the EPA already is running for cover, having issued this statement the day after GM’s 230-mpg revelation: ”EPA has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by GM.”

To make matters worse, Nissan just unveiled its Leaf all-electric vehicle – no onboard recharger fueled with gasoline. The company says it can go 100 miles on a single charge (of course, when you run out, you better be close to a wall socket).

Taking the EPA’s formula using the LA4 driving cycle, Nissan calculated that the Leaf – a four-door sedan much like the Volt – gets 367 mpg, trumping the Volt by a wide margin (apparently Americans can’t even make fuel efficient cars fuel efficient.) Note that the 367 miles per gallon rating is for a car without a drop of gasoline, never mind a gallon, on board.

Luckily for GM, Nissan and the EPA, the public isn’t likely to dive too deeply into the calculus. NBC’s Today Show swallowed the fuel economy rating whole, with no mention of how GM or the EPA arrived at the number. CNN ran the 230 mpg number in their endless news cycle without any reflection on the calculation. As far as the news media seems to be concerned, if the government says it’s true, it must be true. Great.

What the public is more likely to listen to will be the sticker price on these new EVs. GM has floated that the Volt will come out around $40,000. Just by comparison, a 2010 Mercedes Benz C-Class, roughly the same size as the Volt, comes in just over $35,000 nicely loaded.

And therein lies the rub – whether it is flying cars, high-speed rails, or electric vehicles. Worse than not getting what we want can be getting what we want. And now the bill for what we all asked for incessantly – incredibly high fuel economy in a four-door sedan – appears to be coming due.

In truth, I hope the calculation proves to be true through some far more complex equation that takes into account the vagaries of regenerative braking and the leftover 30% battery capacity after the driving cycle is finished. I wish I could explain the number just so I could be more preachy and tell GM’s critics  to kiss Rick Wagoner’s posterior (some folks in the Senate get first pucker), as the Volt was conceived under his watch. Having a vehicle that provided 10 times the fuel economy of today’s average car would solve an extraordinary set of seemingly intractable problems, from Middle East oil imports to climate change and the environment.

But for the public to buy into the hype, the car companies had better be sure they all are speaking from the same playbook. In principle, that’s the EPA’s job, but in fact we don’t buy cars from the EPA (well, sort of, if you think about it). It’s up to GM, Nissan and the half dozen other manufacturers planning all-electric or hybrid electric vehicle production in the next few years to give car buyers something solid that really justifies the added cost of owning an EV.

About the author: Rob Cleveland is CEO of ICON Creative Technologies Group and a co-owner of Grange Kitchen and Bar in Ann Arbor.


  1. By Bob Martel
    August 16, 2009 at 6:09 pm | permalink

    If that 62 mpg figure when running in gasoline mode is correct, then that in and of itself is a pretty impressive figure.

  2. August 16, 2009 at 6:22 pm | permalink

    I agree. There is an interesting dynamic here, though. One might immediately ask what happens while the gasoline motor is engaged and, barring any plug in time, how you can get such good mileage. In other words, why not just put this motor in, get rid of the batteries and electric drivetrain. It turns out that the 100-hp motor charges the batteries which subsequently drive at 160-hp electric motor. Sooner or later, depending on how you drive, the gasoline motor won’t be able to keep up with the electric motor demands, and you’ll run out of juice. So plugging in still is a very good idea.

  3. By Kristin Dziczek
    August 16, 2009 at 9:23 pm | permalink

    GM _is_ putting this engine (a Family 0 I-4) in other vehicles, with a turbo. The Cruze (replacement for the Cobalt) will get it, as will others. MPGs on a turbo 4-cylinder conventional gasoline vehicle are reported to be north of the 35 CAFE standard.

    Interesting math on the 230 MPG. Another criticism is that the EPA cycles are frequently run instrumentation only (I don’t know the specifics on the cycle used for the Volt or Leaf). So when you hear the phrase “your mileage may vary” think A/C, heat, radio, windshield wipers, … drivers often see a 20 percent drop from sticker MPG in real life driving. Twenty percent of 230 is a heck of a lot.

  4. By anon
    August 16, 2009 at 11:51 pm | permalink

    When will people stop drinking the Auto Industry kool-aid? Read “The Myth of the Efficient Car”
    Also the metrics people need for gasoline and diesel vehicles is gallons per 10,000 miles for city and highway driving. Miles per gallon is itself confusing and deceptive.

  5. August 17, 2009 at 12:29 pm | permalink

    “GM has floated that the Volt will come out around $40,000. Just by comparison, a 2010 Mercedes Benz C-Class, roughly the same size as the Volt, comes in just over $35,000 nicely loaded.”

    Yes, but buying a Mercedes doesn’t get you a $7500 tax break, which puts the net price of a Volt at $32,500.

    See: link

  6. August 17, 2009 at 1:27 pm | permalink

    The day this announcement came out, I only needed to hear two numbers to call “bullshit” on this claim.

    1. “Volt gets 230 mpg”
    2. “Volt goes 300 miles on a tank of gas”

    If both these claims were strictly true, that would imply a tiny 1.3 gallon tank. I think my (non-riding) lawnmower holds more than that!

  7. August 17, 2009 at 1:31 pm | permalink

    Good point. The tax credits will certainly help mitigate the overall cost. Hopefully the end buyer will see it that way too. Sadly tax credits don’t drive as much demand as say the cash for clunkers program. Now that kind of incentive would help a great deal.

  8. By STeve Andre'
    August 17, 2009 at 1:53 pm | permalink

    Bob, interesting analysis. I certainly agree that the 230
    figure reeks of kool-aid, but still its a great idea and we
    need to get away from our addiction to oil.

    One thing you didn’t take into account however, is the
    efficiency rate of charging the batteries. It is not 100%,
    so the claims about how far you get with NN kWH from the
    wall socket is going to be skewed. How much I can’t say,
    but if you’ve ever felt a battery getting warm when its
    time to feed it, that heat is wasted energy. Different
    battery technologies exhibit different characteristics,
    so maybe this isn’t a huge problem, but there is some loss
    during the charge.

    We’ll see how well these do overall. My other concern is
    how much extra pollution we’re going to create if 5,000,000+
    cars are suckling on the nations power grid each night. In
    terms of carbon, how much goes a gallon of gas create, vs.
    how much that electric equivalent of a gallon creates?
    (I know this doesn’t take into account the fact that we’d
    be more free from oil, but carbon is carbon).

    –STeve Andre’

  9. By Spencer Thomas
    August 17, 2009 at 7:39 pm | permalink

    What about the charging energy? My (amateur) take is that it CAN come from less-polluting sources. Also, the electric grid is underutilized at night. We’ll never power a car with wind but we can power it with wind or hydro or solar generated electricity.

  10. August 17, 2009 at 9:30 pm | permalink

    Spencer. You’re exactly right here. Detractors will tell you that a shift to electricity will only exacerbate the CO2 output from the coal-fired powerplants creating a net zero effect. But charging your vehicle at night is the optimum time to source underutilized capacity. What’s more, just optimizing the electricity grid alone would make the whole affair more efficient. In the end, it’s harder to regulate millions of tailpipes than it is thousands of smokestacks.

  11. By STeve Andre'
    August 18, 2009 at 2:43 am | permalink

    The utilities might be underused at night, but that still doesn’t mean that enough cars feeding at once isn’t going to be an impact, and pollute more.

    I hope that people start making their own charging stations at home with solar panels. Not cheap, but a 220W panel is under $1000 so a couple of them during suny days could give you some number of miles each day.

    As far as the zero effect goes, I don’t know the numbers to figure out how you pit a gallon of gas against the energy needed to create the electricity, ship it and then charge a battery. And of course, how you create that electricity is important as well.

    –STeve Andre’

  12. August 18, 2009 at 7:22 am | permalink

    Steve, it sounds good in principle, having people create their own microstations, but if that was going to happen it probably would have already. Sadly the cost of electricity still is cheaper coming from the power company, but that could change if electricity companies have to compensate for the CO2 output they create. Those kind of legislative efforts would go a long way in making it more cost effective to put a few solar panels in your backyard.

    As far as the cost to produce gasoline versus electricity, the numbers in the calculations above are the end of the line cost to the consumer. But you are right, there is a huge infrastructure cost in getting the electricity to us. There also is a huge cost in getting a gallon of gasoline to the pump, and sometimes those costs aren’t reflected in the price, but rather in the cost to our foreign policy, our national debt and the stability of our future.

  13. By Bob Martel
    August 19, 2009 at 8:42 am | permalink

    While it’s nice for all of us to think about getting our own home-scale windmills and solar panels, I believe that when you look at the big picture, the economics of scale are the same with these clean technologies as they are with the carbon based ones. For example, I have a propane back-up generator to cover us when the power goes out at our home in Lima Township and that costs me in dollars about five to ten times as much as DTE to generate electricity. I assume that there are equivalent inefficiencies in carbon emissions as well between my generator and a large scale coal plant. I think that we need to push and focus the utilities to install and use commercial-scale clean technologies and leave the home-scale technologies to the hobbyists and early adopters.

  14. By Mike D.
    August 30, 2009 at 9:57 pm | permalink

    Rob, thanks for walking through the numbers. As you note, GM’s figure doesn’t seem to make sense, but I think the EPA is leaning toward counting a kWh as less than its literal energy equivalent in gasoline, thus making the gasoline mpg numbers much higher than what you came up with (and closer to GM’s). Maybe the feeling is that, because electricity could be wind or solar generated, the energy used from electricity counts less than gasoline. It’s all speculation, though. There is no “draft standard” from the EPA yet, so GM is just making things up so far, unless they know something that hasn’t been made public yet by the EPA. Whatever the number, the Volt is an amazing achievement.

    One small correction: in one of your comments, you imply that the Volt’s gasoline engine can’t keep the batteries charged enough to run indefinitely when you say, “Sooner or later, depending on how you drive, the gasoline motor won’t be able to keep up with the electric motor demands, and you’ll run out of juice.” According to Frank Weber at GM (the Volt’s head engineer and exec), the Volt will run as long as you keep refilling its gas tank. The gas engine is strong enough to power the car and keep the battery at 30% or so. You couldn’t accelerate with a full 160 hp continuously forever, but in practice, it’s virtually impossible to use all of a car’s horsepower–or anything close to it–unless you’re at a test track. If you floor it, the battery will dip slightly below 30% to give enough juice to deliver bursts of reasonable acceleration, according to Weber. Even in the theoretical world, if you were trying to, say, drive at top speed up a steep hill for many minutes continuously, and you needed all 160 hp, the worst thing that would happen is that you’d get less power temporarily and it would be as slow as a Prius.

    On another note, I ate at Grange for the first time last night and was impressed by the food. The farm-to-table concept carried through in the salmon; it had a pared down, fresh personality. The artwork on the walls doesn’t work with the restaurant’s theme, though. It looks mass produced and not particularly elegant.