In the first 10 days of July, incentives on the GMC Sierra rose to almost $10,000 per truck. That should worry GM investors. Image source: General Motors.

Is General Motors (GM 0.80%) having trouble selling its full-size pickups?

You wouldn't think so from looking at the sales totals. But GM has been doing some odd things lately. Last month, the General rolled out an unusually aggressive ad campaign that bashed (literally) the aluminum body panels on archrival Ford's (F 0.61%) F-150 pickup. 

The ads didn't do much to dent Ford's sales gains in June. Now it looks like GM began July by resorting to a tactic that has bad implications for its bottom line: It boosted its truck discounts in a big way. 

GM more than doubled its discounts on some of its big trucks

Bloomberg is reporting that GM more than doubled the discounts on some of its big pickups during the first 10 days of July. Citing J.D. Power data, which is generally considered authoritative, the report said that GM's incentives on the Chevrolet Silverado jumped 76% from June levels. Incentives on the Silverado's upscale twin, the GMC Sierra, jumped 147%. 

Big incentives on full-size pickups are par for the course; it's just part of how the market for these trucks works, and the trucks are priced to take the incentives into account. But the J.D. Power data cited by Bloomberg suggests that GM's incentives are now more than just "big": The payouts in the first 10 days of the month averaged $7,962 on the Silverado, and $9,457 on the Sierra. 

The average on Ford's F-150 over the same period was a comparatively modest $4,457, the report said. 

The average transaction prices on these trucks typically runs just a bit over $40,000. The difference between GM's incentives and Ford's might well be the difference between a tidy profit on each sale -- and not much profit at all.

That's worrisome for a few reasons.

What is GM thinking here?

GM said that it had a "sale" at the beginning of July in which it offered 20% off of its sticker prices, and that it expects to end the month with incentives much closer to Ford's. Speaking as a GM shareholder, that's somewhat reassuring. 

But even if these discounts were only limited to part of the month, why do them at all? Why give up much of the profit on some of the company's best-selling (and generally speaking, most profitable) products in a booming market? After all, even if GM's year-over-year comparisons aren't as impressive as its executives might like, GM is still selling a whole lot of pickups in what is still a very strong market. 

The idea was probably to give GM's dealers (and its year-over-year sales numbers) a quick boost by turning some on-the-fence shoppers into buyers. But it seems to go against what GM has been preaching in recent years. 

Why GM investors should be concerned

Over the last couple of years in particular, GM executives from CEO Mary Barra on down have been emphasizing the company's "discipline" on incentives, pointedly putting profit margins ahead of market-share concerns. 

It has been a sea change from Old GM's way of doing things, in which profits were too often sacrificed in pursuit of market share gains. This approach has been an important factor in GM's profitable revival -- and it's an important factor in the investment case for General Motors' stock.

If it really was just a short-term sale, it probably won't make much of a dent in GM's third-quarter profits. But the fact that GM felt the need to do it at all is worrisome.