Retail sales in December are expected to fall 1 percent to 1.31 million vehicles from a year earlier, the consultancies said on Friday.
Incentive spending per unit continued in December, as consumer spending is expected to fall for the sixth straight month, J.D. Power and LMC said.
Average transaction price, however, is on pace to reach $34,292, an industry record.
"The continued growth in transaction prices is allowing manufacturers to offset lower sales with higher revenue," said Thomas King, senior vice-president of the data and analytics division at J.D. Power.
Last week, the National Automobile Dealers Association said new vehicle sales in the United States are expected to drop in 2019, as higher interest rates and rising prices could prompt customers to hold off their car-buying plans.
Reporting by Rachit Vats.