I find it ironic that at a time when more people (here in North America; this is old hat to folks in Europe, Asia and other parts of the world) than ever are paying attention to new cars’ fuel efficiency, the system people have been relying upon for assessing that metric is becoming increasingly obsolete. I’ve been giving this problem some thought over the past few days and quite coincidentally, today came a press release from the Automotive X Prize organizers that recommended the industry, the EPA, etc. use “MPGe” (“miles per gallon equivalent”) to measure fuel consumption.
However, the X Prize folks are missing the point entirely!
Thinking about how to state fuel economy for an electric vehicle, one’s tempted to find some metric that will allow it to be compared directly to a fossil-fuel-driven automobile—hence the motivation for MPGe. I thought maybe a more esoteric measurement might be more suitable for comparing all cars on an apples-to-apples basis; maybe, for example, mass of carbon dioxide per distance driven, say kilograms of CO2 per 100 kilometres, to use an SI illustration. Of course, the amount of carbon emitted to produce electricity varies greatly depending on the means of production: more for coal, less (arguably none) for solar.
Ignoring that “little” complication, using a unit such as kg CO2/100 km is still just playing around with units. Call it MPG, MPGe, litres/100 km, kg CO2/100 km are all pretty much the same—multiply and maybe invert and you can convert from one unit to the other assuming you can agree on the constant conversion factors. Granted, it would be nice if decades from now when all cars are electric if the unit of energy consumption wasn’t based on some strange brew concocted out of fossil-based hydrocarbons sucked out of the ground, but then again we’re talking about an industry that still often cites engine “horsepower” a full century after cars started taking the place of equine-powered transportation.
What the above fuel efficiency metrics miss is the non-linearity of fuel consumption in some new cars. For example, drive a short distance in the new Chevy Volt and you’ll be getting great “MPG” (100+ is their claim), but drive far enough to deplete the battery and you’ll be consuming real, not “effective” gasoline and at an “effective” rate much greater than when the car was running from the electrical charge generated by more efficient means.
In fact, it’s easy to imagine how “your mileage may vary”: someone driving a plug-in gas-electric hybrid solely a short distance to and from work and other activities around town would see fuel consumption possibly a full order of magnitude less than someone regularly driving the same model of car on long trips.
The ideal solution to the MPG-number quandary would be a better-informed consumer who understands the problem of trying to oversimplify what is in fact a complex aspect of automotive performance. Skeptics and cynics would argue that greater consumer sophistication is unlikely to happen (read: people are stupid), or that auto makers will still want a dumbed-down metric that people can easily understand (read: they want to market to people who they believe are stupid).
Maybe a few energy efficiency numbers based on representative profiles might work; for example, one number representing a driving pattern consisting only of in-town commutes and errands, another number representing energy use for long trips… The two would seem to correspond roughly to the “freeway” and “city” mileage numbers currently quoted for cars sold here, leaving us pretty much where we started. Not good enough!
I think the fundamental question most people have is, how much (dollars, gasoline, CO2, etc.) will a given car cost them to drive? In this age of ubiquitous information-gathering I think that’s not too difficult a problem to solve. For example, based on measured data or, less ominously, a sufficiently in-depth questionnaire, it would be straightforward to come up with a semi-accurate prediction of fuel/energy consumption based on someone’s particular driving routes, style, habits and preferences.
Then again, “straightforward” assumes that it would be easy to get industry players and regulators to agree upon a single standard. Given how much they’ve butted heads over a “simple” calculation for today’s MPG figures, that’s taking a lot for granted.