Plug-in vehicle makers are losing out on sales dollars because the mileage ratings don't make a lot of sense. That's what one Venture Beat writer says after buying a Toyota Prius Plug-in and trying to apply the miles per gallon equivalent (MPGe) standard to real-world results. In short, it's difficult to do.

The Prius Plug-in Hybrid's 95 MPGe rating is not only difficult to quantify, but it's "artificial" and "purely imaginary" because the proportion of miles driven by electricity vs. gasoline differs from driver to driver. For instance, while a Mazda minivan costs about 21 cents a mile in fuel, the Prius Plug-in costs 10 cents a mile in gas-powered mode and about 2.5 cents a mile in pure electricity mode. For those with relatively short commutes (and therefore a higher percentage of all-electric driving), savings of as much as $100 a month are possible. Still, with electricity rates not readily available, it's impossible to measure a plug-in's powering expenses (compared to gasoline) until one measures the extra electricity used over the course of a month and adds that to gasoline-spending figures.

Even without MPGe being clear and obvious to everyone, US plug-in sales are increasing substantially. Through the end of July, Americans bought more than 55,000 plug-ins in 2014, up 51 percent from a year earlier (and those figures don't include the Tesla Model S, since that automaker doesn't break out US sales). If Venture Beat is right, then plug-in sales should be growing even faster because the public would have a clearer idea of why those cars can make financial sense.

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