For more than a decade, Kyle Hayes of Corsicana, Texas, was a loyal Chevrolet truck buyer, owning one Silverado after another since he was old enough to drive.

But when it came time to trade in his 2012 Silverado heavy-duty truck last fall, he had a change of heart, opting for a Ram truck.

“The biggest thing was the quality of the interior,” said Mr. Hayes, 29 years old, who works in the farm and ranch trade and mostly uses his truck for business. “It is a whole lot nicer truck. My only complaint is that it doesn’t wash itself.”

The Wall Street Journal reported that for decades, Ford Motor Co. and General Motors Co. have dominated the large pickup truck market in the U.S., each selling over 650,000 pickups a year, with Ram trailing as a distant third.

Now, that is starting to change. Under Italian auto maker Fiat SpA, Chrysler Group LLC’s Ram brand has vastly improved the look and performance of its truck—while aggressively pricing and marketing it against competitors—and is quickly closing the gap, analysts say.

Ram’s run is a headache for its larger rivals, particularly GM, which recently completed costly overhauls of its two large pickups and is trying to get higher prices for them.

Ram’s pickup truck sales shot up 21% last year to 355,700 units. Last year, sales of GM’s large trucks grew 16% to 664,800. So far this year, GM’s new pickups are down compared with a year earlier, while Ram pickup sales are up 24%.

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