GM said pickup sales fell 15.5 percent for its Chevrolet Silverado and 8.5 percent for its GMC Sierra.
Spokesman Dan Flores said GM was holding tight reins on discounts, also called incentives, for Silverado and Sierra. It has 89 days of supply for those trucks, which he said put it in a good position heading into the fourth quarter, historically the strongest for pickups.
"We will be building inventory in the next couple of months," said Flores. "This is an issue we monitor on a daily basis. We're not going to get into a discount war."
FCA's Ram 1500 pickup truck showed a sales increase of 29 percent.
Analysts were expecting September U.S. industry auto sales to fall 0.8 percent to 2.5 percent, a poll by Reuters showed.
Most analysts forecast a decrease in 2016 from last year's record sales of nearly 17.5 million vehicles.
As the market shrinks, automakers are hiking incentives to entice consumers to spend. September industry incentive spending is tracking at an average $3,923 per new vehicle sold, which would surpass the previous high of $3,753 from December 2008.
In contrast to analysts' expectations, Toyota and GM executives said on Monday that a new record was within reach.