Although other factors were in play, the chart show how the leveling off of vehicle weight marks the point at which average fuel economy began its general upward trend--despite a trend of increasing vehicle power. (Chart includes preliminary projections for MY2013.) 

MY2012 was record-breaking for fuel economy and CO2 emissions

Average MY2012 vehicle CO2 emissions fell by 22 g/mi from MY2011, with a corresponding improvement in fuel economy, according to a new report by the U.S. EPA. The figures of 376 g/mi and 23.6 mpg figures are both records, the agency said. The 1.2 mpg fuel-economy improvement was the second largest annual improvement in the past 30 years. Average vehicle weight was 3977 lb (1804 kg), a decrease of 150 lb (68 kg). Average MY2012 vehicle power was 222 hp (166 kW), a decrease of 8 hp (6 kW) from MY2011. Estimated 0-to-60 mph (0-97 km/h) acceleration time in MY2012 was unchanged at 9.4 s. Average vehicle footprint declined by 0.7 ft² (0.07 m²). A decrease in light truck market share was a major factor in the lower weight, power, and footprint.

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