The game wasn't close in the first half of 2015: Chevy Silverado sales went up, and Ford's dropped. But can GM hold on to that advantage? Source: General Motors.

The first half of 2015 is in the books, and two big numbers stand out in the sea of auto-sales results released this past week:

  • Chevrolet Silverado sales: Up 14.6%
  • Ford F-Series sales: Down 2.4%

Fold in the Silverado's upscale twin, the GMC Sierra, and General Motors (GM -0.56%) sold more full-size pickups in the first half of 2015 than mighty Ford (F -0.74%).

The F-Series still outsold the Silverado, and it's still "America's best-selling" truck line. But while Ford has been touting its all-new aluminum F-150, GM has been scooping up sales, market share, and profits.

What's the story?

Ford hasn't had enough F-150s to go around...
It's not that the F-150 is a bad truck. In fact, reviewers say it's a very good truck, and it has been selling very quickly.

The problem is that Ford hasn't been able to make enough of them -- yet.

Ford makes the F-150 in two factories. Normally, it takes just a few weeks to change over a factory's assembly line to the tooling needed to make a new model. But, as I'm sure you've heard by now, the new-for-2015 F-150 has aluminum body panels, not steel like the old truck.

Ford's two factories needed extensive changes to make the new truck, because aluminum requires different tooling and processes than steel. Each of Ford's two factories -- which normally run almost around the clock -- were down for about 12 weeks in total.

That cost Ford a lot of trucks. At the start of this process last year, Ford North America chief Joe Hinrichs estimated the company would lose 90,000 units of production, or about six to seven weeks' worth of sales. 

Ford's new F-150 looks likely to be a big hit, once Ford has enough of them to meet demand. Production has been ramping up slowly for months. Source: Ford Motor Co,

Both factories are now back up and running at full speed. But supplies of the new F-150 have been tight for months. As a result, Ford has given first priority to its most profitable customers: retail buyers seeking loaded trucks. 

Normally, commercial fleet buyers account for a significant portion of Ford's (and GM's) full-size pickup sales. But until recently, Ford was holding off on sending the new trucks to its commercial fleet customers. "Fleet sales" get a bad rap from investors, but commercial fleet sales are good profitable business. 

They're not as profitable as sales of loaded luxury trucks to retail buyers, though. That's why Ford steered its early production to its dealers.

It was a smart move. But it gave GM a big opportunity, and GM doesn't miss many opportunities nowadays.

...which gave GM an opportunity to grab some business
GM doesn't break down its fleet sales by model. (Neither does Ford.) But like Ford, it does tell us what percentage of its total U.S. sales were to fleet customers every month, and it gives us some deeper details from time to time.

Here are two telling details General Motors shared this past week: GM's deliveries to commercial-fleet customers were up 20% in the first half of 2015, and its deliveries of full-size pickups to government-fleet customers "more than doubled" last month versus a year ago.

"Commercial-fleet sales" doesn't just mean pickups. But a lot of pickups are sold to commercial buyers: think contractors, mining companies, oil-field services companies, even cable companies. They all buy a lot of pickups, and most of those trucks come from GM or Ford. (Fiat Chrysler isn't a big player in this market, although it would like to change that.)

The upshot: Both Ford and GM played their hands well, but now the game changes
Here's the long story short: Ford couldn't make enough of its new trucks to go around. So it put some of its business up for grabs -- and GM moved right in to grab it.

But the game could look a lot different in the second half of 2015. Ford's factories are back up to speed, and the company should have full inventories of its new trucks soon, likely within a few weeks.

At that point, Ford's new-for-2015 F-150 will finally be able to go head-to-head with GM's new-for-2014 Silverado and Sierra. What happens then? We'll find out in a few months.