Why We Should Measure by Gallons per Mile, Not Miles per Gallon (2024)

In the debate over how to measure fuel economy, it comes down to this simple tenet: It's easier to make comparisons between the fuel consumption of two vehicles when expressed as fuel consumed per unit of distance, not distance per unit of fuel consumed. Specifically, we should measure gallons of fuel consumed divided by 100 miles traveled, or g/100m. In much of the rest of the world, this is already how it's done, although liters/100 kilometers are the units used.

Calculating fuel consumption in gallons per mile involves two additional steps. Say your car or truck is rated at 18 mpg. Take the reciprocal of 18 by dividing one by 18 and get 0.055, which is gallons per mile. Then multiply by 100 by moving the decimal point two places to the left, and you'll get 5.5 gallons/100 miles.

Those with a particular aversion to algebra can go to the EPA's fuel-economy website, and select their vehicle. Once you have it displayed, you can change the mileage displayed in the data panel from the oh-so-familiar window-sticker mpg to g/100m. You can also simply enter the conversion into the Google search engine as "mpg to gallons per 100 miles" and their calculator does the math for you.

But why, exactly, should consumers care?

Quick, which is better: Replacing an 18-mpg car with a 28-mpg one, or going from a 34-mpg car to one that returns 50 mpg? Researchers at Duke University say that drivers find it easier to select the right answer when efficiency is expressed as gallons per 100 miles (g/100m). So 18 mpg (or 5.5 g/100m) versus 28 mpg (3.6 g/100m)--an increase of 10 mpg--represents a 52 percent reduction in consumption. If you trade in a car rated at 34 mpg for one rated at 50 mpg, its a 16-mpg improvement, so we ought to see those gas card bills plummeting, right? Actually, after a minute's worth of math, you'll get 2.9g/100m in the 34-mpg car and 2g/100m in the 50-mpg car--only half as big a gain as the original scenario.

The gallons-per-mile system makes it easier to see that our efforts to reduce petroleum consumption and carbon emissions should focus on removing from the fleet the vehicles that have the poorest economy, even if we replace them with ones that only return moderately good economy. Spending five or 10 thousand dollars to put a hybrid or diesel powerplant into a Honda Civic will save fuel. For example, the annual fuel cost of a 21-mpg Taurus is $1742, while that of a 28-mpg Focus is $1307--a 7-mpg improvement, netting a reduction in annual fuel costs of $435.

Now consider replacing a 29-mpg Honda Civic with a 42-mpg Civic Hybrid. One might think a whopping 13-mpg bump in economy would save almost twice as much money compared to moving from the Taurus to the Focus. But let's do the actual math: $1263 annual fuel cost for the Civic minus $871 for the Civic Hybrid equals, umm, $392. Nothing to sneeze at, but shouldn't there be a bigger difference?

Lets do that over again comparing g/100m instead of mpg:

>
Taurus: 4.8 g/100m
versus

Focus: 3.6 g/100m

Difference: 1.2 g/100m
Civic: 3.4 g/100m

versus

Civic Hybrid: 2.4 g/100m

Difference: 1.2 g/100m

The difference in the mpg ratings doesn't reflect the actual difference in fuel consumption the way that gallons/100 miles does, pure and simple. And that's why PM is starting to report fuel economy this way on its test drives. Pointedly, the EPA should promote this system too. Sure, it's trivial to calculate the difference, but with the current emphasis that car manufacturers, government agencies and consumers are placing on fuel economy, we need to change our way of thinking from high mileage to low consumption.

(Photograph by Jonathan Kantor/Getty Images)