Trends in Technical Efficiency Trade-Offs for the U.S. Light Vehicle Fleet 2007-01-1325

Technology change can yield some combination of changes in fuel economy, performance, size, or other characteristics that influence the fleetwide fuel consumption. Thus, engineering progress can be manifest in multiple ways, reflected in the view that technical efficiency may improve even when fuel economy does not. This analysis quantifies underlying technical efficiency gains that reflect trade-offs between fuel economy and other attributes of light vehicles.

Prior analyses yielded estimates of technical efficiency trends ranging from 1% per year to nearly 4% per year. Few analyses revealed consistent trends; most identify periods when progress was made at different rates. Using fleetwide (aggregate) data for U.S. light vehicles, exploratory data analysis and correlation techniques were applied to examine trends in energy-related variables. Individual trends for all variables are uneven over time. A ton-mile per gallon metric shows some consistency, but still leaves significant variation unexplained.

Good correlations were found using a Performance Size-Fuel economy Index (PSFI), defined as a product of these three factors. PSFI appears quite linear over 1977-2005, with a straight line explaining 99% of the variation for cars and 97% of the variation for light trucks. The resulting correlations yield estimates for trade-offs among fleetwide fuel economy, performance, and size. The results show generally stronger technical efficiency improvement rates over the past 25 years than previous analyses. The results also suggest diminishing growth rates for fuel economy at higher absolute levels of size and performance. Relative to a 1977 baseline, technical efficiency as measured by PSFI shows an average gain of 5.3% per year for cars and 3.1% per year for light trucks. Linear PSFI trends are found to be consistent with engine specific power trends observed over the period of analysis.