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GM's Chevy Silverado has posted nice sales gains this year. GM is hoping for more with a hard-hitting set of new video ads. Image source: General Motors. 

General Motors (NYSE:GM) took things up a notch this past week in its long-simmering war with Ford (NYSE:F) and its market-leading F-150 pickup trucks.

In a new set of social-media ads, GM takes a series of hard swipes at the new 2015 F-150's aluminum body panels, comparing the new Ford unfavorably to the Chevy Silverado and its "high-strength steel." 

What is GM doing here, and why now?

Is a steel Chevy a more "manly" pickup?
The new ads are clearly tongue-in-cheek -- but they aren't subtle. 

In one, shown below, a group of guys is shown cages made of aluminum and high-strength steel -- and then told they can choose which one they want to hide in when a hungry grizzly bear is released. Later, an announcer touts the strength of the Silverado's steel panels, which one participant describes as "a manly truck."

 

Source: General Motors.

In another ad, a superhero called "Aluminum Man" is presented to an unimpressed focus group. A third, featuring former NFL star Howie Long and a GM engineer, abandons the humor to make the message clear: Aluminum isn't a strong choice for a pickup, and Ford's pickups might be more difficult and more expensive to repair if damaged.

It's an interesting message for GM to be pushing, given that the company is believed to be incorporating more aluminum into its next-generation Silverado, due in a few years.

It's also interesting because GM's full-size pickups have been gaining market share at Ford's expense in recent months. Why would it launch these aggressive and possibly polarizing ads now?

GM's truck sales have been great. Why sling mud now?
GM's recent gains aren't trivial. Through the first six months of 2015, sales of the Silverado rose 14.6%, while sales of Ford's F-Series (which includes the F-150 and its Super Duty siblings) fell 2.4%.

In other words, GM has been gaining a bunch of market share. Fold in strong sales of the Silverado's upscale twin, the GMC Sierra, and GM's share of the U.S. retail market for full-size pickups has risen 0.9 points since 2013 to 38%, according to J.D. Power figures supplied by GM.

But here's the thing: GM -- and its dealers -- know that the game is about to change in a big way.

Why GM's dealers are worried about Ford now
Ford hasn't been losing ground in the sales war because its new truck is a loser. It has been losing ground because of supply issues. In order to manufacture the new aluminum-bodied F-150, Ford had to make extensive changes at its two pickup factories.

The F-Series is America's best-selling vehicle line. Normally, both of those factories run around the clock to keep up with demand. But each needed to be shut down for 12 weeks in order to install and test the new tooling needed to make the new trucks.

The upshot is that Ford has had tight supplies of pickups for months. As a result, it has been prioritizing retail sales, allowing GM room to steal some of Ford's lucrative commercial-fleet business

But now both factories are up and running at full speed. In just a few more weeks, Ford's dealers will have full inventories -- and Ford will get serious about retaking the ground it has lost to GM.

That's why GM is ratcheting up the competitive pressure.

Will GM's preemptive strike really hurt Ford?
Will the ads work? Will GM be able to keep Ford from retaking much of its lost market share?

They probably won't hurt, and their existence is already making GM's dealers happy. But I doubt they'll do a whole lot to help. 

While Ford has downplayed its new truck's fuel-economy advantages in ads, it hasn't been shy about touting other advantages of weight reduction, like better towing capacity.

And buyers seem sold on the new trucks. So far, the new F-150s have been selling very quickly, at very good prices. GM will do its best to defend its turf, but I expect Ford to post some nice year-over-year sales gains in the next few months.

John Rosevear owns shares of Ford and General Motors. The Motley Fool recommends Ford and General Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.