A review of recently available data on both on-road fuel economy and new car test fuel economy shows that while US on-road fuel economy has been flat for almost 15 years, major European countries and Japan have shown modest improvements in response to “voluntary” agreements on fuel economy, steadily rising fuel prices (since 2002), and to some extent shifts to smaller cars and 2nd family cars.

At the same time the sales weighted average of new vehicles sold in the European Union, expressed in terms of their implied CO2 emissions, have fallen short of 2008 goals. That a significant part of the improvements in Japan are related to the growing share of mini-cars (displacement under 600 CC) suggest that technology is not the only factor that can or will yield significant and rapid energy savings and CO2 restraint in new cars.

Fuel economy technology, while important, isn’t the only factor that explains differenced sin tested or on-road fuel economy when comparing vehicle efficiency and transport emissions in different countries. Fuels, technology, and driver behavior also play significant roles in how much fuel is used. As long as the upward spiral of car weight and power offsets much of the impact of more efficient technology on fuel efficiency, fuel economy will not improve much in the future. And as long as the numbers of cars and the distances cars are driven keep creeping up, technology alone will have a difficult time offsetting all of these trends to lower fuel use and CO2 emissions from this important sector.