The good news: the average fuel economy of new cars and light trucks sold in the US has been increasing steadily, reaching 25.4 mpg in January 2015 after being stuck around 20mpg for two decades. That is good news for the climate, and shows that the CAFE rules, which require the average efficiency to reach 54.5 mpg by 2025, are effective.
The bad news: the US is behind. In Europe, the carbon emission of the average new car sold back in 2013 was 127 g CO2/km; that translates to 43 mpg. This beats the EU target of 130g /km for 2015 (that's 42mpg) by two years.