An increase of 0.2mpg in average fuel efficiency of the entire U.S. fleet of cars, SUVs and pickups marks the fourth year in a row the figure has improved, with the current average figure at 20.8mpg (11.30L/100km). But the figure is based on sales projections made by the car manufacturers before the model year began. Since then, fuel prices have swung far above the levels assumed in the projections, meaning the overall average is in reality expected to be somewhat higher than the projections indicate.
Further, a change in the methodology of the study's averaging practices means the laboratory-tested increase in fuel economy is likely to be even better than indicated. The 20.8mpg (11.30L/100km) value places the 2008 year at 1.2mpg below the fuel-economy peak of 22.0mpg (10.69L/100km) that occurred in 1987. But the projected laboratory composite figure of 26.0mpg (9.04L/100km) places the 2008 model year 0.1mpg above the 1987 figure of 25.9mpg (9.08L/100km). The laboratory composite testing methodology has not been changed; only the methods for calculating the 'adjusted' fuel economy.
The dip in the projected adjusted fuel economy since 1987 is partly explained by the increased market share of heavy SUVs, while the improvements over the last four years reflect the trend toward stabilized vehicle weights and declining SUV and pickup market share. Those two factors taken together are why the EPA is expecting the real market results to exceed even the projected fuel economy increase, as conditions return to a less SUV-centric model.
Other considerations relevant to the overall fuel economy trends are the relationship between vehicle weight, performance and economy. Generally, faster-accelerating or heavier cars will burn more fuel. Car have been improving in performance and weight consistently for several decades, but a steadying of average vehicle weight since 2004 with only small increases in performance have combined to help achieve the best fuel economy figures on record since 1993.