True. But what if I asked you to compare 10 to 20 mpg, vs 20 to 40 mpg?
You might say they are equal, because in both cases you double your miles per gallon.
But if we look at gallons per 100 miles, we'd see that's not the case. 10 mpg is 10 gphm. 20 mpg is 5 gphm. 40 mpg is 2.5 gphm.
So you can see that improving from 10 to 20 mpg saves you 5 gallons of gas per 100 miles driven. improving from 20 to 40 mpg only saves you 2.5 gallons per 100 miles driven.
Extra credit: How much better do you need from a 20 mpg car to save as much gasoline as you save going from 10 mpg to 20 mpg?
Answer: You can't. You'd need an electric car, because you have to improve from 20 mpg to infinite miles per gallon;
The gallons per 100 miles measurement makes it easy to see exactly how much gasoline you would save with the improvement; and since most people know how many miles they drive a year, they can quickly and easily figure out how many fewer gallons of gas they would use, which would tell them whether upgrading their car would ever pay off.
Most people assume that if they could improve their car from 50 mpg to 100 mpg, they'd pay off as quickly as, say, improving from 10 to 20 mpg. in fact, you save 1 gallon per 100 miles improving from 50 to 100 mpg. To save the same amount if you have a 10 mpg car (10 gallons per 100), you only need to go to 9 gallons per 100, which is about 11 miles per gallon.
IN other words, in terms of absolute savings, making sure your 10 mpg car is tuned up and the tires are fully inflated (gaining you around 2 mpg improvement) will save you more money each year than converting a 50 mpg car into an electric car.
So, the question is, what do you want to know?
If you know that you are going to drive a certain amount in a year, say 10,000 miles, you can figure out how many gallons of gas you will use by simply dividing 10,000 by the MPG. All very straighforward.
But if you are trying to compare, apples to apples, different vehicles, then consumption figures make more sense.
One question is, “How much is this car gonna cost me?”, and the other is “Which of these cars is gonna cost me less?”
If your concern is affording the vehicle, the first question is pertinent. If you are trying to reduce your consumption for some external reason, unrelated to absolute cost, then the second one is more relevant.
I would submit, that for most Americans, the former question is the one they worry about. They want to figure out how much it will cost and then weigh that cost against their desire, and come to a decision. This reflects the American attitude that the budget does not define the decision, but rather desire.
But for most Europeans, it seems to me, the budget is the dominant factor. Under such constraints, using the consumption figure makes perfect sense, and it would be inconvenient to do things any other way.
So, it is not the way Americans express auto milage that the US National Research Council has a problem with, but rather the way Americans feel about consumption in general. The “Make more, spend more” attitude of Americans infuriates them, because it does not respect their certainty that supply is limited, so we must all limit our desires.