New cars sold in the US had a record-high fleetwide fuel economy, up about 14 percent over the past four years, the Detroit Bureau reports, citing the University of Michigan Transportation Research Institute (UMTRI). New US vehicles sold in 2012 averaged 23.8 miles per gallon, up six percent from the 22.5 mpg achieved in 2011. High gas prices earlier in the year and a broadening range of both smaller vehicles and electrified cars helped push MPG numbers.

In fact, GM says it was the first US automaker to sell more than one million vehicles in the US in one year with fuel economy of at least 30 mpg on the highway. There was a hiccup in the upward trend, though, since December's new-vehicle MPG numbers fell slightly from November's, likely a result of falling gas prices.

Either way, Americans are buying more advanced powertrain vehicles than ever. US sales of hybrids, plug-ins and diesels last year jumped 63 percent to 540,181 units. More impressively, plug-in vehicle sales almost tripled in 2012 to 49,962 units, and that's not including low-volume cars like the Tesla Model S and Fisker Karma.

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