Automakers reported strong December U.S. auto sales Monday, thanks to easy credit access and cheap gasoline, but analysts warned such growth may be unsustainable.
 
Industrywide, sales climbed 10.8% to top 1.5 million vehicles last month, Autodata said. At an annualized rate, sales slowed to 16.92 million from 17.2 million in November. But actual full-year 2014 sales rose 5.9% vs. 2013 to more than 16.5 million.
 
Generous financing is boosting sales figures, while declining driver's license penetration among young Americans should be a drag in the long term, Morgan Stanley analyst Adam Jonas wrote in a note.
 

A new Jeep Cherokee rolls through assembly at the Chrysler Toledo North Assembly Plant in Toledo, Ohio, on May 2014. Getty Images View Enlarged Image
 
"U.S. auto sales are dancing to a very different (and we believe unsustainable) beat," he said, suggesting that auto demand is also outstripping wage and housing growth rates.
 
Longer loan terms are helping more people afford to buy cars. In Q3, the average car loan was 66 months, according to Experian Automotive, up from 62 months five years ago.
 
For now, falling gas prices and discounts are drawing customers to dealerships. Fiat Chrysler Automobiles (NYSE:FCAU) reported a 20% jump in U.S. sales last month. Sales hit 193,261, missing views, but higher-margin gas guzzlers did well. Ram truck sales leapt 32%. Shares fell 3.8%.
 
GM's (NYSE:GM) sales surged 19% to 274,483, easily topping forecasts, despite a string of recalls throughout the year. Silverado truck sales soared 36% last month, and GMC Sierra sales jumped 31%. Shares fell 2%.
 
Ford (NYSE:F) saw its best December since 2005 with sales of 220,671, up 1%, and the company said demand for its new aluminum-body F-150 truck is high.
 
But shares fell nearly 4% after Citigroup downgraded Ford to neutral from buy while upgrading GM to its Conviction Buy list.
 
Citi said GM has an advantage over Ford in the shift to larger automobiles and is less vulnerable than Ford is to a declining yen.
 
Toyota (NYSE:TM) is bullish about 2015 as sales rose 12.7% to 215,057 vehicles in December. Truck volumes rose 15.7% to 106,840.
 
Nissan's (OTCPK:NSANY) sales rose 6.9% to 117,318. Car sales jumped over 12%, led by the Sentra, Altima and Leaf electric car.
 
Honda's (NYSE:HMC) sales edged up 1.5% to 137,281 with strong truck sales helping ease a drop in car sales.
 
As gasoline prices drop, consumers are buying larger vehicles with bigger price tags. Promotions are also still in check.
 
Meanwhile, prices for hybrids and other alternative-fuel cars fell 1.2% to stay competitive, according to Kelley Blue Book.
 
TrueCar (NASDAQ:TRUE) estimated the average transaction price for light vehicles was $33,168, up 1.9% from a year earlier, while the ratio of incentive spending to ATP was 8.7% vs. 8.8% in November.
 
"Sales volume growth and the popularity of highly profitable vehicle segments — pickups, utilities and luxury vehicles — has been immensely beneficial to automakers' revenue, and we expect more good news in 2015," said John Krafcik, president of TrueCar, in a statement.