170? 230? What do all these numbers mean?

You may have seen the recent headlines:

• Dakota County students say hybrid gets 170 mpg
• G.M. Puts Electric Car’s City Mileage in Triple Digits

Everyone is excited about high-mileage plug-in hybrids (GM has run a long, mysterious advertising campaign touting the 230 mpg figure). That's all well and good, but, really, is there a good way to measure energy use in miles per gallon when a fair amount of the energy in these cars is measured in miles but not in gallons? HOURCAR's own plug-in hybrids (oh, yes, we're way ahead of the curve; we got our first back in '06) are advertised as 100+ mpg and, under the right circumstances, achieve that. Sometimes. So, should we take all these new numbers with a grain of salt?

Yes.

As this page has discussed before, mileage estimates are just that: estimates. They are very variable. And the higher the mileage, the more variability will occur. Especially since the EPA has not yet finalized how they are going to measure PHEVs. GM's own Volt site has the disclaimer that

230 MPG and 25 kWh per 100 miles driven represents a preliminary GM est. of the final approved city rating; actual MPG and electricity consumption will vary based on many factors including driving habits, weather and road conditions and operation of electrical systems in the vehicle.

That's a pretty big group of ifs! It depends on the final rating, the actual consumption and many additional factors. Maybe 230 is a best-case scenario. Maybe not. But there's really no way to know.

The news articles, when you read past the lede, are not much more forgiving. The Times says:

The rating number, based on methodology drafted by the Environmental Protection Agency, is somewhat abstract, one auto specialist said, given that much of the city driving of electric vehicles will rely solely on the battery charge.

…

Figures for highway driving and combined city and highway use have not been completed for the Volt, but G.M.’s chief executive, Fritz Henderson, told reporters and analysts at a briefing on Tuesday that the car was expected to get more than 100 miles a gallon in combined city and highway driving.

100 mpg is slightly less than 230, and these numbers are coming from the same folks who, a few short years ago, thought the Hummer was the future of American motoring.

CNN goes to point out that this number has not been vetted:

The Environmental Protection Agency … says it has not tested a Chevy Volt and therefore cannot confirm the fuel economy values claimed by GM.

It goes on to point out that

Fuel economy for hybrid vehicles like the Toyota Prius is displayed in the same way as it is for any other gasoline-powered vehicle. It gets 46 mpg, for example, versus 19 mpg for a V-6 Ford Mustang.

That standard works because all the energy used by the Prius ultimately comes from burning gasoline. The Prius just uses that energy more efficiently than other cars do. The Chevrolet Volt, on other hand, runs on electricity that comes from two sources -- a battery as well as a gasoline engine.

When gasoline is providing the power, the Volt might get as much as 50 mpg. But that mpg figure would not take into account that the car has already gone 40 miles with no gas at all. So let's say the car is driven 50 miles in a day. For the first 40 miles, no gas is used and during the last 10 miles, 0.2 gallons are used. That's the equivalent of 250 miles per gallon. But, if the driver continues on to 80 miles, total fuel economy would drop to about 100 mpg. And if the driver goes 300 miles, the fuel economy would be just 62.5 mpg.

And there is, of course, the question of where all that electricity comes from. If it's from wind and solar, huzzah, it's clean. If it's from coal, as much of our energy is, it's not so clear-cut. Plus, how will a Volt measure up against the upcoming all-electric Nissan Leaf and rumored Prius Hybrid from Toyota? And how do you measure the efficiency—and pollution—put out by an all-electric vehicle?

Dave Friedman, quoted in the San Francisco Chronicle, may sum it up best saying that

Your mileage may vary means more to the owner of a plug-in than any other car on the market.

He also points out that conventional hybrids, paired with better engines and transmissions, will have more of an effect in the short term.

So, what does all of this mean? Well, there are couple of takeaways. The first is that while the future is closer, it certainly isn't here, and any numbers you see you should treat with a healthy level of skepticism (as well as a similar level of excitement). The second is that as we increase overall efficiency, how you drive will have more and more of an effect on efficiency.

Finally, as always, the best way to reduce your carbon footprint is to drive less. The marginal increase in emissions by taking the bus or train is almost zero, since they are running anyway. The marginal increase in emissions of walking or biking is negligible. In other words, it's great if your car gets 100 mpg, but your bike does a heck of a lot better than that.