According to a press release from GM, estimates based on preliminary EPA standards have the Volt getting 230 miles per gallon in city driving. I’m not sure what these “preliminary standards” are, but after running some numbers, I have a hunch (and I’m not the only one): I think the EPA is only counting gasoline burned, and not the electricity the car uses for short city trips. Here’s why.
The LA Times article states that the Volt uses as little as 25 kWh per 100 miles of city driving. This number really impresses me — that’s probably significantly less than my 2003 Prius uses (see below).
However, I have to ask, How much gasoline would it take to produce that 25 kWh of electricity? The energy density of gasoline is about 37 kWh per gallon. So, if the Volt only uses electricity, and if electricity could be produced from gasoline with 100% efficiency, the Volt would get a mileage of at most about 148 miles per gallon.
Of course, it is not possible to convert gasoline to electricity with 100% efficiency. I found this electrical plant that converts fossil fuel (natural gas) to electricity with an efficiency of 58%. A a typical efficiency is closer to 40%. Electrical transmission losses dissipate another 5-10% of the power. So, if, optimistically, my electricity is made by fossil fuel at about 45% efficiency, the actual mileage would be closer to 67 miles per gallon.
I do think that if we must use cars, electric cars are the way to go. Hopefully, in the future, more of our electricity will be made from wind and solar farms, nuclear power, and maybe even fission reactors (I can dream). In that case, the electricity comes with much less pollution, and the Volt is a winner.
Unfortunately, in the near term, when our electricity is made from coal and natural gas, the preliminary EPA mileage estimates for the Volt have little meaning. The 230 miles per gallon number is laughable. The Volt will get better mileage than a hybrid or diesel sedan, but it is not yet a big gain.
Instead, if you really want to make a difference, think in terms of person-miles per gallon (or gallons per mile per person), and carpool!