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.