General Motors has announced that the Chevy Volt will get 230 mpg for city driving, and probably around 100 mpg for combined city/highway driving.
The Volt obtains this performance, of course, through its use of a battery recharged from the grid. “230 mpg” means “230 miles per gallon of gasoline,” and ignores the coal or natural gas which in most cases will supply the recharging. The Electricity Fairy has not been coming around a lot lately.
A proper metric for a vehicle such as the Volt depends on what factors the buyer really cares about…
If your main concern is “energy independence,” then “miles per gallon of gasoline” is probably a reasonable criterion.
If your main concern is operating cost, then you need “total cost per mile,” based on a combination of gasoline cost and electricity cost.
If you worry that the world is going to run out of energy, you should be looking at “BTUs per mile.”
And if you really believe CO2 is going to destroy us all, then the metric you should care about is “CO2 emissions per mile.”
According to this, there are people in EPA who want to calculate the mileage by assuming that the battery is recharged from the car’s internal combusion engine at the end of a driving cycle. That idea seems really preposterous, given that the buyers of this car should be those who will normally be able to “refill” the battery on a regular basis and minimize the ICE use.
It’s probably only to be expected that GM would hype the mileage of this car, but one would hope that journalists would point out the limitations of the “mpg” methodology when a vehicle like the Volt is involved.