Sales of the 2014 Ford F-150 have been up, but some competitors have posted far bigger gains. Photo credit: Ford Motor Co.

It's one of the biggest questions in the auto business: Will Ford's (NYSE:F) long-dominant F-Series fall behind this year?

It's a very important question for Ford investors. The F-Series, which includes Ford's F-150 pickup and its Super Duty siblings, is the most important driver of profits in Ford's North American division -- and that division is carrying all of Ford on its back right now.

But the current F-Series lineup is nearing the end of its life. The all-new 2015 F-150 will be launched near the end of the year. Meanwhile, its three biggest rivals are either all-new or have been substantially refreshed recently. 

Hence the question that analysts have been asking since January: Would the F-Series lose market share in 2014? Or alternatively, would Ford be forced to crank up profit-eating incentives in order to keep pace with its rivals?

After four months, a trend is emerging
It's only the beginning of May, but we can see some trends emerging.

Through the first four months of 2014, sales of the F-Series are up 3.9% over the same period last year. That sounds like a modest gain, but remember: Harsh winter weather kept U.S. auto sales down across the board in the first quarter. 

But has the F-Series lost market share? While total sales of General Motors' (NYSE:GM) all-new pickups, the Chevy Silverado and GMC Sierra, are down a tiny bit -- about 0.6% -- from year-ago levels, sales of Fiat Chrysler's Ram pickup line, which was refreshed for 2013, are up 23% through April. 

And then there's Toyota (NYSE:TM), whose heavily refreshed Tundra is up 18% so far this year. 

The Tundra and the Ram sell in much smaller numbers than Ford's F-Series, which can skew our perspective on their gains. If we add together the gains for GM, Chrysler, and Toyota, we see that as a group they are up 8.3% (or 29,233 trucks sold) over the same period last year, while Ford is up 3.9% (or 8,872 trucks sold). 

So Ford has lost a bit of ground to its rivals. Is this something to worry about?

The secret behind Chrysler's big gains
It's not a surprise that Toyota's Tundra has seen some gains. Toyota did heavily revise the Tundra for 2014, and it has always had a following. But its sales are a fraction of Ford's -- about one-sixth in any given month. Its gains aren't keeping Ford executives awake at night.

The Ram, though, has seen outsized sales gains for several months now. It even outsold the Silverado in March, a feat that Chrysler hadn't managed in 15 years.

Big sales gains for the 2014 Ram have come as Chrysler has offered big incentives. Photo credit: Fiat Chrysler.

But that feat came at a big price. Chrysler's incentives have been very high. Incentives are the cash-back or cheap-financing deals that we often see advertised on TV. Those are funded by the automakers, and Chrysler has been funding a lot of them.

In March, Chrysler's incentives Chrysler's incentives on the Ram 1500 averaged $5,598 per truck. That was 35% higher than what Ford was offering on the F-150, and 46% more than GM's offers on its light-duty pickups, according to J.D. Power figures reported by Bloomberg.

Meanwhile, Ford has made a point of holding its pickup incentives steady, at around $4,000 per truck, varying a bit with the mix of truck models that Ford sells in any given month. (In fact, they fell by $210 per truck in April, to around $3,700.)

Incentives help sell trucks, but they're an expensive tool 
Boosting incentives can make sales jump, especially in the commercial-fleet markets where so many pickups are sold, and where fleet managers tend to be more price-sensitive, and less brand-loyal, than individual consumers.

But incentives cut into profits. Both Ford and GM have been trying to rein in their use of incentives in order to keep profits strong (or in GM's case, to boost them from so-so levels). 

Sales of the upscale GMC Sierra, shown here in premium Denali trim, are up 8.1% this year, even as Chevy Silverado sales have fallen. GM's recent emphasis on premium-priced trucks helped boost profits in the first quarter. Photo credit: General Motors Co.

GM in particular has been very stingy with incentives on the new trucks it rolled out last fall, reasoning that much-improved products should require fewer discounts to sell. The new trucks have done well at the upper end of the market, above $40,000, but they've struggled at the lower end. 

GM added some "strategic" incentives on double-cab and V6-powered models to try to change that, and things improved a bit in April, it said. But it held overall incentives at significantly lower levels than in the past.

In some ways, that strategy has worked out very well for GM. Average transaction prices have been up sharply, and that helped GM's profit beat expectations in the first quarter. It has lost some ground to the Ram (and to Ford), particularly at the lower end of the market. 

But GM is still in the process of launching its all-new heavy-duty pickups, and it's working through some supplier issues that have limited production of certain versions of its new trucks. As those issues are resolved, GM's sales could rise. 

The upshot for Ford
Ford executives often talk about "staying on plan" and not making big changes because of month-to-month actions by competitors. That discipline has helped give Ford strong profit margins over the past couple of years, and it doesn't look likely to change -- yet.

That's why I don't think we need to worry that the F-Series will lose ground to the Ram, at least not yet. As a Ford shareholder, I'd rather see Ford doing what it's doing: staying focused on overall profit rather than small changes in market share.

But if GM, with its much larger monthly sales volume, starts to gain significant ground on Ford, that will be something to worry about. Stay tuned.