energy blog

Chevy Volt's 230 miles per gallon claim?
What does it really cost to drive?

Thomas M Crawford
8/16/2009
revised 9/5/2009

GM recently announced that it will introduce its new Chevrolet Volt, a plug-in hybrid car, sometime in 2010.  It claims the Volt will get 230 mpg (miles per gallon) in city driving, and says it dwarfs the fuel economy of any car on the market today, including Toyota’s Prius hybrid.  How does it do this feat?  During city driving, GM is assuming that the Volt will be almost totally electric, and its gasoline engine will not need to kick in much at all. If you do the calculation right, you get 230 mpg, but it is not taking into account that the owner needs to charge the battery of the car before going for the 230 mpg drive. GM is feeding you some propaganda in saying this. The actual cost per mile of driving the Volt in the electricity only mode might be about 20% better than operating it in the hybrid mode, but by no means is it free (as the propaganda seems to imply).

The reason this claim is propaganda is: You can't get something for nothing! or put another way: There's no such thing as a free lunch! (Physicists rephrase these statements into something called the First Law of Thermodynamics.) If you want to push a car of a certain size and weight down the road and through the atmosphere, it takes a certain amount of energy to do it. Yes you can make cars more aerodynamic and engines more fuel efficient, but there are limits (no matter what Congress or President Obama want to legislate or what Al Gore preaches about - there are no physics tests for getting into politics). And I'm sure that GM hasn't gotten more fuel efficient or more aerodynamic than Toyota, Nissan, or Ford, otherwise they would not have needed a bailout. So what is the real cost of driving a plug-in hybrid like the Chevy Volt?

Cost to drive? Here's my "back-of-the-envelope" analysis:

• The best car I can compare the Chevy Volt with right now is probably the Toyota Prius. (Yes the car that usaually has an Obama bumper sticker that now most people are trying to peal off.)  The Prius is a hybrid, which means electric motors turn the wheels, just like the Chevy Volt. The only difference between a hybrid and a plug-in hybrid is that the latter (the Volt) has a bigger battery which can be charged by plugging it in at home to charge the battery for local driving. The Volt still has a gasoline engine turning a generator for charging the battery when you want to do more than just local driving, operating in this case just like the Prius.
• The Prius is a very efficient car. The EPA claims it gets 48 mpg for city driving.  This means the Prius uses 0.0208 gallons of gasoline per mile in the city (this is just 1 divided by the mpg).  (Note - for the Prius to get 48 mpg, it must be driven exactly the right way.)
• There are about 114,000 BTUs of energy in a gallon of gasoline (from Wikipedia.org), and one KW-hr (kilowatt hour) of electrical energy is equivalent to 3413 BTUs of energy (from Physics I & II by Halliday & Resnick).  These two conversion factors are used to convert the energy in a gallon of gasoline into its equivalent energy in KW-hrs.
• Using these conversion factors, a Prius uses 0.6959 KW-hrs per mile.
• The efficiency of a gasoline engine varies depending on the type of engine used, and how the engine is run. It varies between 25% and 35%. I will assume the gasoline engine on the Prius and Volt is at the upper end (35%) because they will design the engine to operate in its most efficient range.
• Using this efficiency factor, this means the Prius actuallys uses closer to 0.2436 KW-hrs per mile.
• My latest tank of gas (8/11/2009) cost $23.66 for 9.7 gallons or about $2.44 per gallon.
• My latest electric bill (7/2009) was $203.00 for 1277 KW-hrs, or about $0.1590 per KW-hr. (Note - This is my average cost for electricity. Some power copanies have "progressive rates" and charge you more per KW-hr for electricity over a certain threshold. This would make charging your car much more expensive since it would up your electricity usage.)
• In my case then, my cost to drive the Prius would be about $0.05/mile for gasoline, and if I could plug it in to charge the battery, it would cost about $0.04/mile for the electricity. (The Volt will have different costs per mile than the Prius, but the ratio of the two will be the same.)

Neither cost per mile value above takes into account the other costs of driving (tires, insurance, wear-and-tear, etc.).  Batteries do not last forever either, and they are expensive to replace.  Since the Chevy Volt needs a much bigger battery than the Toyota Prius, its battery replacement cost will be much higher.  How often will you need to replace the battery?  I just replaced my conventional car battery in my Honda Civic after four years in the Arizona heat.  Life of a battery in a Prius or a Volt is probably similar, but since they possibly use a different type of battery, life might be somewhat different.

What about Carbon Dioxide? Which mode (all electric or hybrid gasoline-electric) on the Volt produces less carbon dioxide (if you happen to worry about such things)? Driving the car using either mode utilizes a generator generating electricity to charge a battery, and then an electric motor takes electricity from the battery for propelling the car. In the hybrid gasoline-electric mode, a small conventional gasoline engine turns the generator. A short electrical transmission line goes from the generator to the battery, so there is little loss along the transmission line. On the other hand in the all electric mode, a generator possibly hundreds or thousands of kilometers away generates the electricity to charge the battery. There is significant loss along this electrical transmission line. If the power company's generator uses a gasoline engine to turn it, it will require more gasoline to generate the amount of electricity needed to charge the battery, therefore more carbon dioxide is produced. An oil plant, a natural gas plant, or a coal fired plant probably also produces more carbon dioxide to charge the car than would be produced in the gasoline-electric mode. On the other hand, a nuclear plant, a hydroelectric plant, or a wind turbine generating the electricity would not produce any carbon dioxide at all. So the answer to this question depends on the power plant used to generate the electricity, and therefore probably whether you are charging your car at a peak power time, or during a time when there is excess power on the electrical grid. It also depends on where in the world you live.

Bottom line - two conclusions can be drawn from the above analysis:

• The Chevy Volt costs a similar amount per mile whether it is being driven in the all electric mode or in the hybrid mode. A Volt will not save you much on fuel in the all electric mode.
• The Chevy Volt is not necessarily more green to operate than a comparable gasoline-electric hybrid, or possibly even a small efficient non-hybrid gasoline powered car. So if you are buying a Volt to feel good about saving the planet, enjoy your feeling of well being, but the all electric mode is possibly more polluting than the hybrid mode..

One more comment - though I did this analysis for the Chevy Volt, the exact same analysis applies to all electric vehicles. If we want to start running our cars on electricity instead of hydrocarbon fuels, we need to start cranking out nuclear power plants to drive down the cost of non-carbon dioxide producing electricity. (Neither wind nor solar can produce enough "green" energy to handle everyone moving from gasoline to electric vehicles.)