As the price of oil spiked to $150/barrel, car shoppers and the media began focusing on fuel efficiency ratings. For gasoline-powered vehicles, the number that is important is the ‘Miles per Gallon’ (or ‘Kilometers per Liter’ outside of the US and UK), the distance one unit of gasoline takes you. The United States EPA has a protocol that it follows to test the MPG ratings of various automobiles, providing a convenient way for making comparisons.
However, when considering electric vehicles, the calculation is not nearly so straightforward. Many car manufacturers are providing a “blended” fuel efficiency number, resulting in ratings of 100 MPG or even higher. They do this by assuming that drivers will only travel slightly more than the electric range of the car. So, if you own a hybrid whose battery provides 30 miles after each charge, and you only drive it 32 miles each day before plugging it back in, the amount of gasoline that is used is minimal. This type of (arbitrary) scenario provides the justification for the MPG numbers being circulated in advertisements and the media.
It is misleading for those who plan to drive their automobile further than its electric range. Additionally, many plug-in models boast of having driving modes that combine the internal combustion engine with the electric motor, providing greater performance, yet using gas even when the battery has a remaining charge.
A more sensible approach would be to offer an electric range (in miles or kilometers) based on the battery alone, and then an MPG rating once the battery is dead. Further tweaks might need to be made for those vehicles that have solar roofs or those that have batteries that can be recharged by using the internal combustion engine. What is clear though is that more transparency is needed for these products. Unfortunately, the EPA has not yet formed a process to regulate the claims of electric cars, and the media has not examined the justification behind some of the numbers being released.
Until some sort of regulation is adopted, or the industry agrees on a standard, consumers need to be armed with knowledge before buying electric cars with inflated fuel efficiency ratings.
You can also read further about why MPG is an imperfect measure when comparing the fuel-efficiency gains between two cars. Gallons per Mile, or GPM, is actually a more direct way to calculate fuel savings.
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