Ford has begun building its all-new 2015 F-150 at its Rouge plant in Dearborn, Michigan, but it'll be months before its dealers are fully stocked with the new truck. Tight supplies of pickups are costing Ford sales. Source: Ford Motor Company.

Ford (NYSE:F) said on Monday that its U.S. sales rose 1.3%, as production constraints on the company's all-new pickups continued to hold it back in a strong market.

Ford's gain missed Wall Street's forecast, which called for a 2.8% increase. It was also far behind the 19% gain posted by rival General Motors (NYSE:GM) and the 20% increase for Fiat Chrysler (NYSE:FCAU).

A big part of Ford's relatively poor performance has to do with supply constraints around its full-size F-Series pickups -- constraints that are allowing GM and FCA to make big gains at Ford's expense.

A big month for GM and FCA as Ford is (again) left behind
GM's full-size pickup twins, the Chevy Silverado and GMC Sierra, handily outsold Ford's F-Series in December, 81,273 to 74,355. Add in GM's new midsize pickups, the Chevy Colorado and GMC Canyon, and the gap widens by another 5,570 trucks.

FCA's Ram was well behind at 44,222 units sold -- but that represents a 32% year-over-year increase for the well-regarded Fiat Chrysler pickup. For the year, Ram sales were up 24% over very strong 2013 results.

In a call for analysts and media on Monday, Ford U.S. sales chief John Felice hastened to point out that while F-Series sales in December actually declined slightly (0.3%) from year-ago totals, its sales were still quite strong in an absolute sense. That's true: It's only the sixth time since 2006 that monthly sales of Ford's full-size pickups exceeded 70,000 units. 

But the real story for Ford is one that has been playing out for several months now: It's missing out on a big boom in pickup sales because its supplies are constrained. The company lost 12 weeks of production at one of its two pickup-truck factories in 2014, as the Dearborn Truck Plant went through an extensive refitting to make the all-new 2015 F-150 -- and now its other factory, in Kansas City, is closed for the same reason.

The new F-150 has been rolling out of Ford's Dearborn factory since November, but production is still ramping up -- only about 5% of the F-Series pickups sold in December were new 2015 F-150s, Felice said. The new trucks have been selling very quickly, but the total sales numbers aren't yet significant.

So is GM doing this with massive incentives?
It doesn't look like it. Industrywide spending on incentives for full-size pickups was actually down a bit in December, according to Ford sales analyst Erich Merkle. Ford's continue to be lower than its rivals' -- that's part of how Ford is managing its inventory during the factory changeovers -- but its rivals' incentives haven't gone through the roof.

Instead, it looks like GM (and FCA) are simply stepping up to take advantage of Ford's supply constraints -- just as a big drop in fuel prices might be drawing more buyers to the segment. 

The bad news for Ford is that it's probably going to lose a year's worth of incremental sales gains once all is said and done. Ford North America chief Joe Hinrichs has said that the factory downtime needed for the changeover to the 2015 F-150 will cost it 90,000 sales -- more than the roughly 77,000 gained by GM in 2014. 

But the good news for the Blue Oval is that by summer, Ford dealers should be well-stocked with the newest pickup on the market. That's when we'll really find out how well the all-new F-150 competes with the GM twins and the Ram.