U.S. auto sales tumbled in October to their slowest pace in four years, as carmakers battled a hangover from incentive-driven sales throughout the past year.

Sales of cars and light trucks fell 27 percent from their record levels of a year earlier. A big drop from that earlier boom was foreseen. But October's 15.5 million seasonally adjusted annual selling pace was even weaker than expected and the lowest monthly selling rate since August 1998, when General Motors Corp. was struggling to refill dealer lots after a summer factory strike, according to the Wall Street Journal.

Detroit's Big Three automakers also lost market share despite heavily promoted discount-financing deals and cash rebates, while Japan's Big Three -- Toyota Motor Corp., Honda Motor Co.

and Nissan Motor Co. -- together gobbled up 3.4 percentage points of additional U.S. market share, according to the Journal.

NADA Chief Economist Paul Taylor said one reason for moderating sales is the very strong performance in July and August, which pulled some normal fourth quarter sales into the third quarter. But stronger incentives also create new customers, and the year should still end with sales of around 16.8 million units, according to Taylor.

Fourth quarter incentives will turn some typical used-car buyers into new light vehicle

buyers, according to NADA. And positive real GDP growth of near 1 percent, combined with

enhanced customer incentives, will help keep sales fairly strong, the dealer organization said.