The news that the the new Chevy Volt is going to get an estimated 230 MPG was plastered all over the internet this week. I'm going to analyze how GM came up with the Volt's mileage rating and provide the method that gives the best way to truly arrive at an electric car's mileage rating. A great analysis of this 230 MPG rating can be found on this Scientific American post and in the comments, where several readers had a great dialog going.
GM came to the 230 MPG conclusion by assuming that drivers will mainly stay below the 40 mile battery range, and using the gasoline portion of the engine rarely. As many of the articles have pointed out, the method that the Volt's current mileage is calculated could result in an "Infinity" MPG, as long as the driver never drives over the 4o mile battery range. Well technically my current gasoline powered car also can have an incredibly high MPG (As of yesterday I was getting 25.7 MPG which is a combination of city/highway/and erratic driving to avoid crazy Atlanta drivers who never use turn signals). My 6 cylinder gas powered sedan has a mileage of 1,000 MPG......as long as I am always driving down a very steep incline and never use the gas pedal.
An example of one way in which GM has arrived at the 230 MPG rating can be seen in my spreadsheet calculation below. In this scenario I assumed that for the first 40 miles the Volt solely uses battery power (the Volt's batteries can store 16 kWh worth of energy). The next step was to come up with a value I thought was fair for the Volt's mileage under the power of its gasoline engine. Since my current GM model sedan gets between 25-26 MPG over a range of city/highway driving, I think a fair assumption of the Volt mileage under gasoline power would be around 30 MPG. The actual value could be lower because their gas engine powers the batteries, which power the electric motor. I'm not sure if there is a loss in efficiency doing this, but I'm assuming so. It might also be slightly higher than 30 MPG. That's the great thing about the spreadsheet, is that users can manipulate the "GM calculated mileage" however they see fit.
I then combined the miles driven under battery only and added it to the miles driven under gas power and divided that total by the estimated mileage of the gas powered engine. I chose 6 miles driven under gas power just so I could get the 230 MPG rating that GM came up with. Feel free to manipulate this as you see fit.
There are obviously a few flaws with this calculation. First, the MPG rating doesn't take into account the amount of energy stored in the batteries and where that energy came from. The current MPG rating just tells you how much gasoline derived energy you used, not total "fossil fuel" energy. If you were to buy the Volt for environmental reasons, you should look at Miles per Emissions released or something along those lines (combining the amount of CO2 released creating electricity for the batteries + the amount of CO2 released in the gasoline engine).
The other flaw is that GM is making too many assumptions in their calculations. They're assuming that drivers will be doing most of the driving under 40 miles, thus rarely having to rely on the gasoline engine. While this could be true, it doesn't make it easy to compare apples to apples (the Volt vs. other cars). In order to compare the MPG of plug-in electric cars with gasoline back up vs. electric hybrids vs. gasoline only, there is only one fair way to do it (please take note EPA).
The best way to calculate mileage of cars using different technologies is fill up the gas tank, drive till the tank is empty, record how many miles you went, then divide the total number of miles by how much gas your tank can hold (according to the comments in the Scientific American post, the Volt has a 12 gallon gas tank). This should be done in many different scenarios (car is full of people and luggage, car just has driver, car is all city driving, car is all highway driving, etc.). Then the total MPG of all those scenarios should be averaged out. This will give drivers a realistic assessment of their true gasoline mileage.
The total distance the Volt can go under gasoline power is estimated by using an assumed mileage and multiplying that by the size of the gas tank. I did this in the spreadsheet above. Using my 3o MPG estimate and the apparent size of the Volt's tank of 12 Gallons, the Volt could go 360 miles under gasoline power. Plugging this distance driven under gasoline power into the spreadsheet above results in the Volt's total mileage (electric + gasoline) of 33.3 MPG, which is a far cry from 230 MPG!!!
In actuality, if you really want to compare cars that derive some or all of their power from electricity, you eventually have to move away from Miles per Gallon and use a method that doesn't rely on gasoline; such as Miles per CO2 emitted or Miles per $ spent on energy (which would be a combination of how much you paid for gas + how much you pay for electricity). More on that to come.
I'm not trying to bash the Volt, in fact I'm thinking about getting one in a few years. It looks like a great car and I love the concept of a plug-in electric. I just want to make sure that buyers are able to do a fair analysis themselves so they don't have to rely on a manufacturer's marketing department to tell them the mileage!
***Update - 10/03/09 - Martin LaMonica over at CNET's Green Tech blog has a great post about plug-in hybrid mileage. The article is about how the SAE (Society of Automotive Engineering) is suggesting that the car is listed with both electricy/mile and gallons/mile, which I think is a very wise idea.