By all measures, 2015 was a blowout year for global automakers in the U.S.

New-vehicle sales, in terms of units, broke an all-time record for the year, probably reaching 17.5 million units in light of December totals reported by large U.S. manufacturers early Tuesday. The previous annual record of 17.4 million vehicles came in 2000.

Fiat Chrysler (FCAU - Get Report) , led by the Jeep brand, posted sales of 217,527 vehicles for the month, up 13%. That's the sixth straight annual increase for the company, which was created following the 2009 bankruptcy of Chrysler.

Nissan (NSANY) reported a 19% increase for the month.

General Motors (GM - Get Report) , the industry leader, said December sales rose 5.7% to 290,230. GM also was reorganized following bankruptcy.

Toyota (TM - Get Report)  said sales rose 10.8% in December.

Ford (F - Get Report)  reported vehicle sales of 239,242 last month, an increase of 8.4% from a year earlier.

LMC Automotive and J.D. Power & Associates had forecast industry sales for the month to be the best in more than a decade.

Average vehicle pricing also represented a peak, with several categories -- especially high-priced luxury, sports and performance models -- registering large gains. According to Kelley Blue Book, the average transaction price of a new vehicle in December was $34,428, up 0.7% from a month earlier.

"Prices continue to climb, reaching record levels, with more new product on the market than ever before, and new product iterations tending to bring higher transaction prices, particularly among luxury models," said Akshay Anand, analyst for Kelley Blue Book.

Honda (HMC - Get Report) , for example, notched a 1.5% average transaction price increase last month over the previous month on the strength of its new Civic sedan and Pilot crossover models. Pilots were selling for an average $37,242, an 11% increase from a year earlier when only the outgoing model was available.

The boost to the nation's economy from automotive commerce was evident in the projection of $58 billion of revenue from new vehicles realized during the month, reported by TrueCar, the digital vehicle purchasing platform.

"This revenue figure is a remarkable conclusion to a historic year for auto sales," said Eric Lyman, TrueCar's vice president and analyst. "Looking ahead, automakers are in an excellent position to kick off 2016. Interest rates remain low, the employment outlook is strong and Millennials continue to enter the market in greater numbers, supporting our expectation of 18 million new vehicle sales this year."

December's results portend strong financial results for the quarter when automakers begin reporting later this month.

Following a historic collapse of auto sales in 2009 after the global financial crisis, the U.S. automotive industry steadily has gained steam. As buyers flock to larger, better-equipped and more costly vehicles, they've also piled up considerable debt in order to afford their purchases.

With interest rates rising and unit sales increasing in record territory, the pressing issue for automakers is how much longer demand will stay strong and how much discounting can they afford when demand inevitably weakens. Growing inventories of unsold vehicles usually are a reliable indicator of a sinking automotive market.

Doron Levin is the host of "In the Driver Seat," broadcast on SiriusXM Insight 121, Saturday at noon, encore Sunday at 9 a.m.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.