One disadvantage of living in a country stuck using non-metric measuring systems is that we’re also stuck using a few related measures. One of the most familiar of these is mpg, miles per gallon. Many other countries use a newer standard, l/100km, liters per 100 kilometers. The salient difference between the two is not merely that one is metric and the other is not; it’s that they are multiplicatively inverted from one another. One gives fuel per distance, while the other gives distance per fuel.
When we talk about a car’s fuel economy, what we want to know is how much fuel does it use, not how far does it go. mpg answers the latter question, while l/100km is what gives us the answer we want. When we talk about a car that gets great fuel economy, we tend to speak instead in terms of how far it will go. This is the inverse of what we are trying to communicate.
The problem with using mpg — distance over volume of fuel — is that the relationship is not a flat line. It’s deceptive. Values at the lowest end of the mpg scale — where most of us scoff at all such cars — correlate to consumption numbers that are much further apart than those at the high end of the scale.
For example, imagine two cars that get 14 mpg and 17 mpg. Most of us look at them and say they’re both unacceptable. However, consider two cars that register 33 mpg and 50 mpg. Who among us would not clamor for the 50 mpg car, believing it to be far more fuel efficient than the 33? The fact is that in both pairs of examples, the car with the higher figure saves 1 full gallon of fuel on a 100-mile trip. Going from a 14 mpg car to a 17 saves exactly as much fuel (and carbon) as going from a 33 to a 50.
The advantage in using a linear scale of fuel consumption is that it tells us exactly what we want to know, without disguising the reality behind an invisible curve.
An increasing number of Monroney stickers — those mandatory window stickers on new cars that include the mileage among other things — are beginning to show l/100km values, though it’s still in a small font below the larger combined EPA mpg number. Still, it’s a step in the right direction.