The truth behind 230 MPG - Chevy Volt's marketing scheme

Chevy Volt 230 MPG?

Chevy Volt 230 MPG?

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 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.

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Ok, here's my simplistic idea: Publish everything MPG, but based on the cost of the electricity. Therefore: I assume that the 16kWh of battery storage is completely used to go 40 miles. So at 21 cents/kWh, this is $3.36 to go the 40 miles. With gas at $2.99/gallon, this would be 1.12 gallons to go 40 miles. So, we have the gone the equivalent of 40 miles / 1.12 gallons = 35.6 MPG, ON BATTERY POWER ALONE! This is not so good!
But how would you handle the different electricity/gas prices across the U.S.? Another option would be to calculate based on CO2 emissions, but those vary for electricity based on what the generation source is and how far away a user is from the generation.
Tom, good comment, but I still feel the MPG calculation method is flawed. I do agree with you that electric motors are a much better method of powering our vehicles and my next car will be a full electric or electric plug-in hybrid. It will be much easier to clean our energy supply if we can just focus on generation and the smart grid. Instead of having to solve that problem along with transportation fuel (i.e. hydrogen/ethanol/compressed air/methanol/etc.) By using electric motors for transportation, you just have to focus on the supply part.
another calculation:
Chris -- I haven't dug into this, but based on my reading, the calculation is a lot simpler: <em>for the typical driver whose commute is less than 40 miles</em> the mileage is calculated simply based on the energy delivered to the battery from a single (plug-in) charge. Since the car would run entirely on electricity, the calculation is simply the conversion of electrical cost per watt to the power available in a gallon of gasoline, given reasonable assumptions about the cost of each. So yes, the calculation isn't real, but then again, it's also not as outrageous as some would have us think. It is indeed surprising, or was to me, how little electricity is needed to propel a car forward using an electric motor as compared to a gas motor. Gas is wildly less efficient (a good deal of its energy is lost as heat), and the motor itself has some limitations that electric motors do better with (e.g. initial torque needed for acceleration from a stop). The real fallacy here is in the need to compare the costs of electricity and gasoline in arriving at a comparable MPG figure. I pay between $0.17 and $0.21 per kWh for electricity, and about the same as everyone else for gas. But do these costs represent the full costs of the product? It depends, and is also somewhat unknowable in the specific detail. For one, electricity tends to be produced with coal as a fuel, having lots of CO2, compared to gasoline which is cleaner ... unless you account for the costs of refining and transporting crude oil into gasoline, which also depends on a lot of things. And, as it happens, my electricity is generated by wind, which screws the whole thing up. My point is simply this: while GM's marketing department seems to have survived their recent near-death, their lies are not quite as bad as everyone with a pencil and calculator seems to be saying. Their methods make a lot of assumptions, but also are as accepted as any other when it comes to comparing "miles per gallon" of electricity to gasoline. If we are to consider a future where electric cars play a more prominent role, then their batteries may also play an important part in buffering and storing the irregular flow of electricity generated from renewable sources. But an electric motor still is a very efficient and effective way of creating locomotion in vehicles. Almost all freight trains have been diesel-electric for decades, mainly for this reason. Tom

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