Chevy Volt 230 mpg claim is misleading

On August 11, 2009 GM made media headlines by claiming that using EPA methodology its Chevy Volt hybrid vehicle was capable of getting a city driving fuel economy rating of 230 miles to the gallon. That’s 98 km/l or 1.02 l/100 km to those of us on the rest of the planet who use the metric system. The next day the EPA poured cold water on GM’s claims: “The EPA has not tested a Chevy Volt and therefore can’t confirm the fuel economy values claimed by GM.” Relatively few articles took the trouble of dissecting GM’s claims for plausibility.

In reality any mpg figure for this type of vehicle is essentially meaningless because unlike mpg figures for other cars it is highly dependent on how far one drives the Volt between recharges. Volt uses a lithium ion battery with a theoretical capacity of 16 kWh that powers the car for about 40 miles (64 km), depending on driving conditions. Once the battery reaches its lower charge limit, a 4 cylinder gasoline engine kicks in to power a generator to provide electricity for driving. GM calls this internal combustion engine (ICE) the “range extender”.

Do less than 40 miles between charges and the Volt won’t burn any gasoline. Its mpg rating would be infinite, because its only fuel is measured in kWh and shows up on your electric utility bill. Once you exceed the 40 mile limit you will start burning gasoline at a yet unknown rate. The Wikipedia article on the Volt mentions a figure of 50 mpg, almost the same as the third generation Toyota Prius. I am a bit skeptical about that number, given the Prius uses an efficient mechanical transmission that connects the engine directly to the wheels via planetary gears, while the Volt first converts the mechanical power from the engine into electricity and then an electric motor converts the electric power back into mechanical power. Neither process is 100% efficient. Also, at 170 kg the Volt’s lithium ion battery weighs some 125 kg (280 lbs) more than the Prius’ much smaller 45 kg nickel metal hydride (NIMH) battery. This weight difference is not exactly going to help the Volt match the Prius’ fuel economy in city driving, where weight is a major determining factor.

For argument’s sake, let’s assume that the Volt does indeed get 50 mpg while running on the engine, after 40 miles on battery power. So what’s the total test distance in GM’s calculation that it used as the basis for its claim? The portion run on gasoline would be 50/230 of it and the 40 electric miles would be the remaining 180/230. From that we can calculate that the total distance is about 51 miles (40*230/180), of which 11 are on gasoline. You would get 230 mpg only if you happen to go 51 miles between recharges. On the other hand, it could be 83 mpg at 100 miles between charges or even 2550 mpg at 41 miles. Pick your number 😉 It really won’t tell you anything until you also factor in your driving patterns and the cost of domestic electricity for recharging where you live.

Americans basically like big numbers and a figure of 230 mpg sure is eye catching, but it doesn’t really tell you much until you study all the details. Here’s another big number: $40,000. That’s about how much GM is going to charge for the Volt from late 2010 or early 2011, when it’s supposed to go on sale. $15,000 more than a 51 mpg (EPA city rating) Prius III is tough to justify economically: Even at $5 per gallon it would buy 3000 gallons or probably around 120,000 miles at a conservative 40 mpg and no electric bill. It remains to be seen how the brand new lithium ion batteries in the Volt will hold up over time compared to the tried and tested NiMH batteries used in the Prius for the last 12 years. The Prius batteries are backed by an 8 year warranty and there are cars that have done 400,000 km (250,000 miles) on the first traction battery.

The 230 mpg claim is dishonest. They could simply say: “It doesn’t use any gasoline for about 40 miles and after that it gets 50 mpg (or whatever number).” That wouldn’t be too hard to understand for anyone and wouldn’t raise any unrealistic expectations. GM doesn’t even mention what fuel economy the car gets while running on the range extender.

I have to agree with those who charge that GM designed the Volt less as a viable competitor in the low-carbon automobile market than as a clever insurance policy to make a bailout at US tax payers’ expense more palatable to the public. Its technology sounds exciting, but it’s a farce. The main piece of new technology that goes into the car – its lithium ion batteries – will be made by LG in Korea. The rest of the car is basically the same platform as the Chevrolet Cruze and its European sibling, the Saab 9-3.

Let’s remember that Toyota launched the first generation Prius in Japan back in 1997. GM didn’t see the writing on the wall then: Even two years later it went out and bought the Hummer brand. Over the following decade it saw its own market capitalization drop from over $50 billion to essentially zero and would be dead by now but for the assistance of politicians too scared to see GM and its supply chain fail while the country was still heading into the worst recession in decades. Keeping the Volt alive all this time made political sense for GM, whatever the real merits of the project.

11 thoughts on “Chevy Volt 230 mpg claim is misleading

  1. 230MPG is all GM aka government hype. I drive a VW TDI Jetta and get 42+MPG. It too cost over $15,000 less than the estimated cost of the Volt. I calculate the cost per mile for the jetta is about 6.14 cents / mile at $2.579 for diesel. Volt claims it cost 3 cents per mile. Do the math and to recover the $15K takes over 490,445.85 miles to brake even. The Volt as I said is all hype.

  2. I think you missed one other bit of exaggeration from Chevy. Nobody is calculating what those first 40 miles cost. The energy in the batteries is not free – it has a monetary cost to the homeowner as they plug in the car and a planetary cost when the energy is generated. Whether or not 40 miles worth of electricity is better than gasoline depends on where you live and how that electricity is generated. If you live near a big hydro damn, maybe it’s basically free energy. If you live in a rural area in a state that is served mostly by old coal power the initial generation is painful and then the loss due to transmission means that it’s pretty bad for the environment.

  3. @greggles,

    I agree with you that neither the cost of the electricity nor the CO2 output from power generation should be ignored.

    Still, it seems that even if all the power were to come from coal fired power stations, the CO2 output will be no worse than from a conventional gasoline engine car (non-hybrid). For example, an article in International Business Times (Electric Cars Not Yet Free of CO2 Emissions, 20 Feb 2009, http://www.ibtimes.com/articles/20090220/electric-cars-not-yet-free-co2-emissions.htm) compares CO2 output from two electric cars (Tesla, Mini E) with two hybrids (Prius, Insight) and a conventional car (Camry). Even running on coal power the electric cars do better than the conventional car, though slightly worse than the hybrids. It must be remembered that coal generators achieve efficiencies of around 40% (better than for most car diesel engines) which makes up for the higher carbon content in coal versus hydrocarbons.

    We don’t know what the final electric highway power usage of the Volt will be, but the quoted power usage for city driving (25 kWh per 100 miles) is some 15% higher than the highway power usage of the Tesla and 7% higher than the Mini E.

    Highway power usage is likely to be higher than urban (as is the case for the Prius and other hybrids, unlike conventional cars). If we add some 20% to the Tesla and Mini E figures, which is where the Volt is likely to be, we’re in the vicinity of the Camry on CO2 output. That’s the worst case scenario.

    This means the Volt is likely to be equivalent to a conventional car on CO2 output if *all* of its power came form the most CO2-intensive power source. In reality a bit less than half of US power comes from coal. The second most common power source is natural gas, which produces only roughly half as much CO2 per kWh compared to coal. Hydro and nuclear are nominally CO2-free, if one ignores construction of the plants and (more importantly) mining and fuel enrichment in the case of uranium. Still, average CO2 output per kWh is likely to be significantly lower overall than the pure coal scenario at which the Volt would not save much if any CO2 over a conventional car.

    The above “back of an envelope” calculation ignores subtle details such as CO2 output from refining, from trucks that supply gas stations, transmission losses in the grid and energy usage to produce lithium batteries. It will take a lot more effort for a full and accurate comparison.

  4. I appreciate you taking the time to do some “back of the envelope” calculation. Regarding some of the subtle details, I believe that about 68% of energy is lost in transmission (According to http://www.usablemarkets.com/2009/02/11/fascinating-yet-somewhat-confusing-chart-of-us-energy-flow-usage/ ) It’s kind of a crazy number to be that high, but 26.3 / 38.2 is 68%…

    This is obviously a major part of why I think we can’t ignore the CO2/energy cost from electric vehicles.

    IMO, the most important thing is to get a car that gets great mileage and create lifestyles where you need to drive it as little as possible (e.g. live close to work, recreation, and shopping).

  5. The 68% (or 26.3/38.2%) you quote is mostly waste heat in thermal power plants. The bulk of the energy is not lost in transmission but during generation, i.e. in hot exhaust gases going up the smoke stack or waste heat from condensing steam at the low pressure end of a steam turbine.

    Only a small fraction of that 68% is transmission losses in transformer stations and high voltage lines.

    When I wrote that coal fired plants are about 40% efficient, the other 60% go up the smokestack or the cooling tower or into a river as waste heat. That’s the bulk of your 68%.

    Nuclear plants have slightly lower efficiencies than coal fired stations, more like 33% instead of 40%. I think it was because the nuclear fuel elements can only stand limited heat stress, compared to burning coal.

    Combined cycle power stations burning natural gas on the other hand can hit 60% and more. It’s a gas turbine whose waste heat drives a steam turbine, but these still make up a relatively small proportion of power generation.

  6. The good thing about the wild claims is that it gets everybody talking and crunching numbers and thinking. That is what we need. Plus with each step in technology there are some good and bad to go along with it, but we do continue to advance, which is what is important. Not every car will suit everyone’s needs, I could exist 90% of the time with an all electric Geo Metro 4dr with a range of 10 miles, but the other 10% of the time I need a minivan to load up.

  7. Even if it gets exactly the same mpg as the prius it is AMERICAN MADE. I believe the car will help the American economy a lot more than a darned prius or any othe Nipomobile.

  8. @Harry,

    you’re obviously feeling very patriotic. Your choice of words indicates you have a problem with Japanese cars, for whatever reasons. I have never chosen cars based on which countries they’re from, instead judging by reliability, ergonomics, performance, etc. of particular brands and models. Out of curiosity, if you were Serbian, would you have rooted for the Yugo?

    To what extend the Volt really can be called “American made” is a matter of definition. Of course final assembly does occur in the US, by which criterion it is as American as US models of the Toyota Camry 😉

    The body shell and the internal combustion engine of the Volt will be made in the US too, so the percentage of US labour that goes into it is substantial. On the other hand, if it’s really going to be priced at around $40,000 as most people seem to expect, that would put it as just over twice the price of the 2011 Chevy Cruze whose platform it shares.

    The major difference between the Volt and the Cruze is going to be the set of lithium batteries as well as a an electric motor and a generator. The other main part of its drive train, the engine driving the generator (once the batteries are drained) is actually going to be virtually the same as a 1.4 litre turbocharged engine used in the 2011 Cruze (minus the turbocharger) and both engines will be made at the same factory. That engine is a fraction of the price of the battery.

    By far the bulk of the cost that drives up the consumer price by some $20-22,000 from the $18-$20,000 of a Chevy Cruze is going to be the battery. While final assembly of the battery will happen in the US, the lithium ion cells will be made by LG Chem Ltd. in South Korea, who developed the batteries. There is nothing wrong with sourcing the major component from a foreign supplier who (unlike GM itself) has the technology. However, it would be a distortion of the truth to simply label the Volt “American made” when *at least 50%* of the value that went into making it was added by a Korean company.

    Most likely the other just under 50% of the car’s value include some imported parts too, as GM shares a global parts bin with it’s foreign subsidiaries Opel in Germany and Daewoo in Korea.

    I grew up in West Germany, across the border from East Germany, which gave its consumers little choice but to buy (East-)German. Their cars were absolute junk and disappeared as soon as the wall came down, along with about 90% of their other manufacturing jobs. The best way to stimulate the economy is vigorous competition, not economic nationalism. May the best products win!

    I hope the Volt will be a good product and wish GM well, but I dislike the hype, which will only lead to disappointment and loss of trust.

  9. Pingback: Chevy Volt would do 230mpg | Cars Technology - About vehicle of the future

  10. I understand where you are coming from. However, we do need to develop technology in order to get out from under the petroleum czars thumb. To me they are far worse than the car companies you are being negative about. We need to look to the future and develop alternative systems that will replace gasoline.

Leave a Reply

Your email address will not be published.