Why the Chevy Volt shouldnt be rated at 230 mpg

The Chevy Volt’s projected 230-mpg city fuel economy rating has caused quite the commotion since GM made the announcement Tuesday. Whether it’s excitement or skepticism that’s stirring people, the number had resounding impact for one reason: 230 mpg is more than four times greater than the most fuel efficient car on the road today. The Toyota Prius’ 51 city mpg is absolutely paltry compared to the Volt’s rating.

But we’re not buying it.

No, we’re not questioning GM’s credibility here. Details on how engineers arrived at 230 mpg are scarce. Neither the EPA, GM nor the Society of Automotive Engineers have been willing to share the current proposed test methods with us. Still, we’re confident GM is using legitimate draft procedures that will strongly shape the final test process.

For a vehicle like the Volt, though, 230 mpg doesn’t make any sense. In fact, any mpg rating that attempts to combine the efficiencies of the electric and gasoline powertrains doesn’t make sense for a range-extended electric vehicle like the Volt. Some consumers, who diligently charge their Volts and drive less than 40 miles, may never fill their cars with gas. And others, who drive on cross-country road trips, may only see 40 gas-free miles before covering the next 2000 miles with the help of the gas engine. For those very long drives, the real-world mpg number will effectively become the fuel economy of the gas engine – maybe around 40 mpg.

So we’ve now got Volts returning infinity mpg and 40 mpg. And somehow we’ve decided to slap a big 230-mpg sticker on them.

One thing that is clear about the fuel economy procedure GM used is that it included a Utility Factor. This Utility Factor is in place to account for exactly how people drive their range-extended electric vehicles. It takes questions like “How far do drivers travel before recharging?” and boils them down to an average behavior. This allows the manufacturer to translate the electric range and the gasoline engine fuel economy into a single mpg value.

The problem here is that very few of us are actually the average driver. This discrepancy means drivers can expect to see fuel economy numbers that swing wildly among different owners, and for a single driver traveling different distances. Once the gasoline engine kicks on, the Volt’s fuel economy begins to plunge from a very tall peak with every mile you drive, until it reaches equilibrium at the efficiency of the engine.

The graph above (at the top of the light yellow bar) is a simplified illustration of how that fuel economy diminishes. For this explanation, we’re assuming the Volt covers 40 miles on a battery charge, at which point the gas engine kicks in. The driving continues at a constant speed and load, with the gasoline engine running at 40 miles per gallon. At 50 miles, the driver has used just 0.25 gallons and achieved 200 mpg. But just 30 miles later, fuel economy has plummeted to 80 mpg. This is the basic behavior for all range-extended electrics, and will lead to dramatically varying real-world mpg numbers for people as the gas-powered miles pass. Fuel economy drops below 50 mpg after this theoretical Volt drives over 200 miles.

Further confounding the situation, the Utility Factor is currently based on driving behavior studies of the American fleet that were conducted years ago. That fleet was (and still is) composed primarily of gasoline-powered cars. One can imagine that many Volt owners will recognize their car’s primary value in short-range driving, and alter the way they commute, run errands or take road trips.

Nissan tried to temper the Volt’s 230-mpg announcement with its own high-mileage candy. The company says its all-electric Leaf will earn 367 mpg. This claim is even more asinine than GM’s. The Leaf will never suck a drop of gasoline, instead drawing propulsion energy from the electric grid. Sure, you can convert the Leaf’s electricity consumption into a mpg figure, but that has very little relevance for the consumer.

So what’s the solution? We say that buyers should be left to make their own assumptions about driving behavior with a new fuel economy label, as seen above. This new label would be divided into a section for electric performance and gasoline performance. Shoppers could see the range of the electric powertrain in the city and the highway, along with the fuel efficiency of the engine once the charge was depleted. Those who never plan to drive outside of their city can ignore the gasoline portion, and those who do serious highway driving, should read the label with greater emphasis on the fuel economy numbers.

In addition to range and fuel economy information, buyers are provided with key details on the vehicle’s charging time and cost, along with an estimated annual cost that accounts for both electricity and gasoline. As public charging stations are added that operate at higher voltages, this panel could expand to include those charging times. Rather than guessing how a consumer will use their car, the EPA will now enable the customer to determine if range, charge times, costs, and true fuel economy are acceptable to them.

Sure, our proposed label strips GM of its triple-digit fuel economy crown, but it also offers the company an opportunity to market the Volt as a car that never needs gas. Additionally, it provides consumers with the government-certified information they need to confidently purchase a car that meets their expectations and needs.

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