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The current Chevy Silverado is built using steel body panels. Rival Ford has moved to aluminum, but GM may have a different plan for the next Silverado. Source: General Motors.

Ford (NYSE:F) has been slow to ramp up production of its all-new aluminum F-150 pickup. But so far at least, demand for the new truck appears to be very strong.

That's good news for Ford. The Blue Oval, mindful of tough new fuel-economy regulations that will go into effect in a few years, gambled that its truck buyers would accept a pickup that wasn't made of the traditional steel. It looks like they have.

General Motors' (NYSE:GM) current full-size pickups, the Chevrolet Silverado and GMC Sierra, were all-new for the 2014 model year. Unlike the new Ford, the GM twins have steel body panels.

But those tighter fuel-economy rules will apply to GM, too. It's a safe bet that the next generation of GM's big pickups will need to be considerably lighter than the current models.

But GM has already begun running ads suggesting that Ford's aluminum truck is flimsy. So how will its next trucks compete with the aluminum F-150?

One way or another, the new Silverado will probably have to be lighter
To be clear, we don't know much about GM's next-generation pickups. My best guess, based on some industry sources, is that they'll be 2019 models,. I suspect that GM will officially unveil them early in 2018, probably in Detroit. But officially, we don't even know that much. GM might well choose to launch them earlier.

What we do know is that current U.S. fuel-economy regulations will require Detroit's pickups to get much better fuel economy starting in a few years. That means the new trucks will probably have to get lighter: All other things being equal, a lighter vehicle uses less gas.

That was the reason behind Ford's move to aluminum. It was an expensive move, because it required big changes to Ford's two truck factories.

The new F-150's body panels are bonded -- glued, essentially -- and then riveted in place. The old F-150's steel panels were welded and bolted into place. The factories needed new tooling and procedures in order to make the new trucks. Those changes meant weeks of downtime; Ford said up front that it would lose 90,000 units of production as it made the changes.

GM said last week that it was investing in some changes in one of its truck factories. They're the kinds of changes you might make if you were planning to build an aluminum truck in the future. It is a safe bet that GM is making the changes now, without shutting down the factory, in hopes of avoiding the long stretches of downtime that Ford went through later on.

But I'm not expecting GM to build bonded-and-riveted aluminum-bodied trucks like Ford. I think GM has a different plan for its next pickups, and I think they're already testing it out -- on a Cadillac.

How a new Cadillac will help GM build better pickups
In the last year or so, GM product chief Mark Reuss and senior GM engineers have talked up what they call a "mixed materials" approach to reducing the weight of their vehicles. At the same time, they have hinted that they are skeptical of Ford's bonding-and-riveting approach.

As an example, the upcoming new Cadillac CT6 sedan will use a mix of lightweight steel alloys, aluminum, and carbon fiber composites in order to reduce weight without compromising safety or rigidity. It's more complicated than simply switching to aluminum body panels. 

I have heard hints that the high-end CT6, which will likely sell in low numbers, is GM's way of testing and perfecting methods of manufacturing a "mixed materials" vehicle before it rolls it out on bigger-selling (and less expensive) products. Put another way, I think they'll be using the knowledge gained from building the CT6 as they gear up to build the new trucks.

I don't think the new GM trucks will have all-aluminum body panels like the F-150, though it may have some aluminum panels (the hood, for instance). I suspect that GM wants to make the most of the new trucks' body panels out of some (perhaps lighter-weight) form of steel, not least because it'll continue to give its marketing folks plenty of ammo to sling at Ford. It'll also help hold off GM's other big truck rival, Fiat Chrysler. FCA hinted last year that it might switch to aluminum when the Ram is overhauled in 2017, but a more recent Reuters report said that FCA is likely to stick with steel in hopes of winning over more commercial customers.

I think GM will also stick with steel, at least to some extent. That's partly for marketing reasons. But I also think that GM, which has (among other innovations) patented techniques for welding aluminum to steel, genuinely thinks that it can build a better truck that way.

Will buyers agree? We'll find out in a few years.

John Rosevear owns shares of Ford and General Motors. The Motley Fool recommends General Motors. It recommends and owns shares of Ford. 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.