New UMTRI report on US vehicle fuel economy from 1923-2013 suggests need to focus on improving lower tails in each vehicle class
In a new report, researchers at the University of Michigan Transportation Research Institute (UMTRI) have calculated the actual, on-road fuel economy for the entire fleet of all vehicles in the US, and for different classes of vehicles, with primary interest in light-duty vehicles (cars, pickup trucks, vans, and SUVs), between 1923 and 2013.
The main findings from the analysis are that fuel economy decreased from 14.0 mpg (16.8 l/100 km) in 1923 to 11.9 mpg (19.6 l/100 km) in 1973. Starting in 1974, fuel economy increased rapidly to 16.9 mpg (13.9 l/100 km) in 1991. Thereafter, improvements have been small, with fuel economy in 2013 at 17.6 mpg (13.4 mpg US). Overall, the results suggest, the authors conclude, that efforts on improving fuel economy should focus on the lower tails of the distributions of fuel economy in each vehicle class.
|On-road fuel economy of vehicles from 1923 to 2013. Source: Sivak and Schoettle. Click to enlarge.|
The new study by Dr. Michael Sivak and Brandon Schoettle updates a 2009 analysis of actual, on-road vehicle fuel economy in the United States from 1923 through 2006, extending the data through 2013, and including retroactive adjustments made to several values by the US Department of Transportation.
By category, they found:
All on-road vehicles. From 1923 through 1935, on-road fuel economy stayed approximately constant at around 14 mpg. Starting in 1936, fuel economy gradually declined, falling to its lowest level, 11.9 mpg, in 1973. In 1974, fuel economy began to increase rapidly to 16.9 mpg in 1991. Thereafter, improvements have been small.
Cars. On-road fuel economy decreased from 1936 to 1973, followed by major improvements from 1973 to 1991. Improvements since 1991 have been small, with fuel economy at 23.4 mpg in 2013.
All trucks. For all trucks combined, the on-road fuel economy decreased from 10.2 mpg (23 l/100 km) in 1936 to 7.8 mpg (30 l/100 km) in 1965.
Light trucks. On-road fuel economy of light trucks increased rapidly from 9.7 mpg (24 l/100 km) in 1966 to 17.0 mpg (13.8 l/100 km) in 1991. However, the improvements since 1991 have been small, with fuel economy at 17.2 mpg in 2013.
Medium- and heavy-duty trucks. Since 1966, on-road fuel economy of medium- and heavy-duty trucks has improved from 5.6 mpg (42 l/100 km) to 6.4 mpg (36.7 l/100 km) in 2013.
Broadly, although fuel economy decreased from 1935 to 1973, this does not imply that powertrains for vehicles did not improve during this period, Sivak and Schoettle noted. Engineers were focused on increasing power and acceleration rather than fuel-economy improvements.
As has been argued eloquently by Larrick and Soll, equal absolute increases in miles per gallon result in greater fuel savings as the initial fuel economy of a vehicle decreases (despite what most of us intuitively believe).… This observation, however, does not necessarily argue that we should focus our efforts on those classes of vehicles that currently have the lowest fuel economy, such as medium- and heavy-duty trucks, and buses. For medium- and heavy-duty trucks, the relevant societal measure is not miles per gallon but ton-miles of freight per gallon. Alternatively, the relevant measure for buses is passenger-miles per gallon.
The above observations suggest that our focus should be on the lower tails of the distributions of fuel economy in each vehicle class. In other words, society has much more to gain from improving a car from 15 mpg to 16 mpg than from improving a car from 40 mpg to 41 mpg. Similarly, the benefits to society are greater from improving a truck from 4 mpg to 4.5 mpg than from improving a truck from 7 mpg to 7.5 mpg (while keeping the load-carrying capacity the same).—Sivak and Schoettle
Michael Sivak and Brandon Schoettle (2015) “On-Road Fuel Economy of Vehicles in the United States: 1923-2013” UMTRI-2015-25