Which is more useful to know: How far you can drive on a gallon of gas? Or, how much gas you will use while owning a car?
MPG answers the first question. It is useful when judging the range of one's gas tank. But it answers a less important question. GPM answers the question of gas consumption. We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions. MPG cannot.
People rely on subtraction when comparing MPG, which creates illusions. The improvements from 10 to 11 MPG, 16.5 to 20 MPG, and 33 to 50 MPG all save the same amount of gas over a given distance (e.g., 100 gallons per 10,000 miles). Given the inverse relationship between MPG and GPM, MPG requires division before subtraction (e.g., 1/20 - 1/16.5 or 100/20 - 100/16.5). GPM makes the magnitude of gas savings clear without additional math. GPM allows car buyers to use subtraction to compare the fuel economy of different cars (e.g., 500 vs. 400 gallons per 10,000 miles).
Providing a column of GPM numbers at Consumer Reports and at fueleconomy.gov would make accurate fuel economy comparisons far easier than the current column of MPG numbers. GPM needs to supplement MPG as a measure of fuel economy.