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post #1 of (permalink) Old 12-30-2010, 04:10 PM Thread Starter
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General Motors Explains the Chevrolet Volts Fuel Economy Rating

In August last year, General Motors announced something seemingly ludicrous: that its 2011 Chevrolet Volt was likely to get a 230 mpg city rating. That was long before the EPA testing methodology for electric vehicles was set, which caused GM to retract that statement in July this year and to eventually wait for an official EPA rating before shipping Volts to dealers. Now that the EPA rating is set and in the windows, heres a look at how it was determined.

So, by now youre all likely familiar with our Automobile of the Year Chevrolet Volt and how its unique series/parallel hybrid powertrain setup works, but if youre not, heres a recap and a breakdown of how the driving modes were scored by the EPA.

Initially the Volt has a fully charged battery, giving it a range between 25 and 50 miles depending on how the car is driven. This all-electric range is officially estimated as 35 miles by the EPA. In this mode of driving, the Volt uses no gas whatsoever, potentially allowing owners to only use one tank of gas per year (the Volt senses when gas is going bad and will burn a tank once per year to keep the engine healthy).

The EPA decided to give consumers a figure relating the electrical efficiency to something they understand already, so it converted electrical efficiency to an equivalent gasoline efficiency Miles Per Gallon equivalent, or MPGe. To obtain a MPGe rating, the EPA determines a miles per kilowatt hour rating and then uses the energy in one gallon of gasoline (roughly 33.7 kWh) to convert this mpkWh figure to MPGe.

After the battery is depleted a gasoline engine kicks on to act as a generator, supplying energy to the drive motor. This is the second MPG figure given by the EPA and is straightforward. When the gasoline engine is running, the Volt is rated at 37 mpg combined.

Of course, most Volt owners will likely use a mix of the two driving modes, which caused the EPA to add a separate addendum. Another portion of the Volts EPA sticker provides a relative breakdown of the costs per mile owners can expect under a variety of conditions. The EPA breaks down charging every 30, 45, 60, 75 miles, and never. This new rating system turns something potentially incredibly confusing into something, well, slightly less confusing.
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