In the past few months, I have highlighted how General Motors Company (NYSE:GM) is poised to benefit from falling oil prices and pickup truck supply constraints at top rival Ford Motor Company (NYSE:F).

GM is one of the biggest beneficiaries of falling oil prices in the auto sector.

This dynamic emerged in full force during the month of December, as General Motors outpaced the auto industry by a country mile with U.S. deliveries up 19% year over year. Most important, GM's sales mix and incentive spending will both contribute to strong profit margins for GM North America.

Full-size pickups and large SUVs lead the way
Two of the most profitable segments of the U.S. auto market are full-size pickups and large SUVs. These vehicles tend to have very high average transaction prices and produce high margins.

Traditionally, General Motors is very strong in these segments, and it has updated its entire portfolio of full-size trucks and large SUVs in the past two years. Moreover, demand for these vehicles is very hot right now thanks to the strengthening economy and the rapid decline of gasoline prices in the past four months.

As a result, dealers posted strong growth in vehicle deliveries last month across virtually all of GM's full-size truck and large SUV models. In total, December deliveries of GM full-size trucks and large SUVs rose 32% year over year. These highly profitable models accounted for 61% of GM's total growth in deliveries.

Chevy Silverado sales have taken off in the past few months. Photo source: General Motors Company.

In the full-size truck market, GM also benefited from Ford's ongoing model transition. The highly anticipated 2015 Ford F-150 went on sale last month, but inventory will be very tight until Ford gets its second truck plant up and running to build the 2015 models, which will happen in the next few months.

Ford sold 74,355 F-Series trucks last month, which is nothing to be ashamed of. That was down just 0.3% year over year, but it still was one of the best sales months for the F-Series in the past five years. However, while Ford's full-size truck sales stagnated, combined sales of GM's Chevy Silverado and GMC Sierra soared from 60,447 in December 2013 to 80,823 last month.

GM North America profitability should rise
Last fall, GM reiterated a target of boosting the GM North America pre-tax margin to 10% by 2016 from 8.4% in the previous four quarters. Strong sales of GM's most profitable vehicles could allow the General to hit this target a year early -- leaving aside any potential recall or legal costs related to the ongoing ignition switch probe.

For example, GM reported that average transaction prices hit a record of $36,300 last month: up more than $1,000 compared to November and up about $3,000 compared to December 2013. Additionally, incentives declined from 11.2% of ATPs in November to 10.4% of ATPs last month.

GM's strengthening sales trend and the favorable mix shift to more expensive and more profitable vehicles will likely allow GM to report solid margin growth for Q4. This momentum should also carry into 2015, as U.S. inventories are down about 1.5% year over year despite GM's higher unit sales.

Wall Street analysts are already projecting that GM's EPS will surge more than 50% in 2015, to $4.28. If low fuel prices lead to a prolonged period of high demand for larger, more profitable vehicles in the U.S., GM could easily exceed that forecast. This could send General Motors shares rocketing higher this year.