Sales of Chrysler's Ram pickup have trounced the market in 2014 -- just as they did last year. Source: Fiat Chrysler Automobiles.

What vehicle has seen sales soar 21% this year -- on top of a 21% gain in 2013?

What vehicle has also gained almost 3 points of market share in the last year -- in one of the most lucrative (and fiercely contested) automotive market segments in the world?

If you guessed Fiat Chrysler's (NASDAQOTH: FIATY) Ram pickup line, you guessed correctly. Sales of Chrysler's Ram have long trailed those of rival vehicles from Ford (NYSE:F) and General Motors (NYSE:GM), but it has gained a lot of ground recently -- and it shows no sign of slowing down. 

What's driving this boom for Detroit's No. 3 pickup line? Here are three reasons Ram sales have soared over the last couple of years.

Reason No. 1: It's a very well-regarded truck
Ford's F-150 has a huge fan base and always wins lots of accolades. GM's Chevy Silverado and GMC Sierra were all-new and hugely improved for 2014, and critics have been pleased.

But the Ram has won plenty of fans on its own merits. It was Motor Trend's Truck of the Year in 2013 and 2014. It's also Consumer Reports'  top-rated full-size pickup, beating not only Ford and GM, but Toyota and Nissan as well in the magazine's strict ratings.

Dealers have told me that the Ram wins over a lot of buyers on test drives, where people are often surprised by how smoothly it drives. Consumer Reports says the Ram 1500 is "the most civilized full-sized pickup on sale," and calls it "surprisingly luxurious and refined," with a "smooth ride" and a "whisper-quiet" interior.

The Ram 1500 EcoDiesel has the best fuel economy ratings among light-duty full-size pickups. Source: Fiat Chrysler Automobiles.

It also gets good fuel economy, thanks in part to an excellent eight-speed automatic transmission designed by ZF and built by Chrysler under license. It shares its design with a transmission that ZF supplies to some of the world's top luxury-car makers -- and it turns out to work well in a brawny full-size pickup, too.

Last but not least, Chrysler rolled out a diesel engine option for the Ram 1500 early this year. Rams with the 3.0 liter "EcoDiesel" V6 are EPA-rated at 28 miles per gallon on the highway, best among full-size pickups -- with a 9,200-pound towing capacity. It's an expensive option, but Chrysler says that about 10% of Ram 1500s are now sold with the diesel -- a strong sales rate.

Reason No. 2: Chrysler has been aggressive with incentives
Last year, when GM launched its all-new Chevy Silverado and GMC Sierra, the company said that it would sharply reduce its "incentives," those cash-back and cheap-financing deals that you hear about in TV ads.

Incentives are part of life in the full-size pickup segment, often averaging $4,000 or more per truck. They help sell more trucks to price-sensitive buyers like commercial fleet managers, but they erode profits -- and GM's then-CEO Dan Akerson was determined to boost the company's profit margins in North America, which have lagged Ford's.

For the first several months of its new trucks' lives, GM was very restrained with incentives. Sensing an opportunity, Chrysler boosted its payouts and made big gains at GM's expense. GM's incentives have since risen, but Chrysler has kept its Ram incentives at a level higher than rivals'.

Trucks loaded with luxury features have helped boost average transaction prices for Ford and GM. Chrysler has moved to keep pace with models like this 2015 Ram Laramie 1500. Source: Fiat Chrysler.

Last month, Chrysler's incentives on the Ram averaged $5,163 per truck, versus $4,224 for the Chevy Silverado and $4,049 for the F-Series, according to J.D. Power data reported by Automotive News. 

That level of spending won't help Chrysler's profits, but it has likely helped its sales quite a bit.

Reason No. 3: Chrysler is taking advantage of issues at Ford
Ford's current F-150 is in the last few months of its life. An all-new 2015 F-150 is expected to start arriving at dealers before the end of the year.

Normally, when a vehicle is in the last months of its life, its maker raises incentives -- puts it on sale, essentially -- to clear out the last of its inventory before the new models arrive. But Ford hasn't done that yet, because supplies of the F-150 are a little tighter than Ford would like right now.

The issue is that the all-new F-150 will have aluminum body panels, a huge change for America's best-selling pickup. It's a huge change in more ways than one: Unlike steel-bodied vehicles, which are bolted and welded together, aluminum body panels are "bonded" with a high-tech glue and then riveted. 

Ford's all-new 2015 F-150 will have aluminum body panels and a host of new high-tech features. Ford is making extensive changes to two factories in order to build the new truck; those changes have cost it thousands of units of production. Source: Ford Motor Company.

It's a completely different process that requires completely different tooling -- and extensive changes to the two Ford factories that make its pickups. Ford North America chief Joe Hinrichs has said that Ford will lose 90,000 units of production as a result of the factory downtime needed to complete the changeover.

That's equivalent to about a month-and-a-half of sales. In order to make sure that its dealers don't run out of F-150s before the new models arrive, Ford has actually lowered its incentives in recent months. That has reduced sales while keeping Ford's profits strong -- which was Ford's plan -- but it has also created an opportunity for competitors, one that Chrysler has moved aggressively to seize.

Will the Ram's hot streak continue?
Chrysler seems to think it will. The company announced this past week that it had redesigned the assembly line at its Warren Truck Assembly plant to boost Ram production by over 28,500 trucks a year.

The Ram may well continue to gain ground. But it seems unlikely that that Ram will be able to post the kinds of market-trouncing sales gains we've seen over the last couple of years for much longer, for a few reasons. 

First, Fiat Chrysler CEO Sergio Marchionne has hinted that the company's incentives will be reduced in coming months. Second, GM is getting more aggressive with its own incentives, as it also moves to take advantage of short F-Series supplies. Automotive News has reported that GM recently launched a new program of bonus payments to Chevy dealers who hit ambitious sales targets for the Silverado.

Last, but definitely not least, there's an all-new F-150 coming from Ford shortly, it's packed with high-tech features -- and it's a safe bet that Ford will market it very aggressively. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.