Unexpectedly Intriguing!
February 15, 2012

We recently tapped the U.S. Department of Transportation's Traffic Volume Trends report to use the data it provides on the total miles accumulated by Americans on the nation's roads as a measure of the health of the U.S. economy, but it occurred to us that we can also use the data to get a sense of how the overall fuel efficiency of the nation's road vehicles is changing over time!

Since the DOT's data only covers the period since January 1986, our first chart shows the number of miles accumulated by all U.S. ground-based vehicles in the 12 months prior to the reported month:

Total U.S. Vehicle Miles Driven During Previous 12 Months, January 1986 - November 2011

Next, we went into the U.S. Energy Information Administration's data on the amount of motor gasoline, distillate (aka "Diesel") and residual fuel oil distillate to find out how much of these petroleum products were supplied to American vehicles for each month since January 1986:

Total Gallons U.S. Finished Motor Gasoline, Distillate and Residual Fuel Oil Product Supplied, January 1986 - November 2011

So now that we have the number of miles driven by Americans each month (which we can approximate by dividing the rolling 12-month figure provided by the U.S. DOT by 12), and also the number of gallons of motor gasoline and distillate products supplied to Americans, which we'll assume all went into the nation's vehicle gas tanks, we can just divide the miles by the gallons to find the nation's average fuel economy for each month since January 1986. Our results are below....

Effective U.S. Vehicle Fuel Efficiency*, January 1986 - November 2011

In January 1986, the average mileage of vehicles traveling on U.S. roads was 10.5 miles per gallon. That figure rose then to roughly 13.0 miles per gallon in January 1993.

From there, it took another 15 years to rise just one more mile per gallon, reaching roughly 14.0 miles per gallon in January 2008.

Since then, the apparent fuel economy of the nation's vehicles has increased a bit more rapidly, rising by almost 0.75 miles per gallon between 2008 and 2009, but falling back from 2009 to 2010, but rising to nearly 15.0 miles per gallon from 2010 through November 2011.

Something Doesn't Add Up

We though we'd compare our results with those of the U.S. Environmental Protection Agency.

Here, we found a 2010 report that covers the period of time from 1975 through 2010, which provides specific data for the average fuel economy of all U.S. cars, trucks, vans, etc. for 1975, 1988 and 2010.

Looking at the years of 1988 and 2010, which our data overlaps, we found that the average fuel economy of all U.S. vehicles was 21.9 miles per gallon in 1988, which increased by 2.7% to 22.5 miles per gallon in 2010.

Keeping in mind that our data likely includes petroleum products that go into places other than the fuel tanks of U.S. ground-based vehicles, we see an average increase of nearly 32% in the nation's apparent fuel economy, rising from 11.0 miles per gallon in 1988 to 14.5 miles per gallon in 2010.

If we're right in suspecting that the combination of finished motor gasoline and distillate fuel products is going into much more than just the fuel tanks of America's cars, trucks, vans, and so on, what that apparent difference suggests is that almost all of the apparent gains in the nation's effective fuel efficiency have taken place outside of the nation's ground-based vehicles!

So much for those federal CAFE standards, right?! The more interesting question perhaps then is who's really not consuming anywhere near as much finished motor gasoline and distillate fuel products today as compared to 1988? As best as we can tell from the EPA's data on vehicle fuel economy, it's somebody other than the nation's drivers!

And that's something for which we don't yet know the answer!

Data Sources

U.S. Department of Transportation. November 2011 Traffic Volume Trends. November 2011.

U.S. Energy Information Administration. U.S. Product Supplied, Petroleum and Other Liquids. Accessed 14 February 2012.

U.S. Environmental Protection Agency. Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975 Through 2010. November 2010.


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