The consultancies say monthly auto sales to be reported next Tuesday by major automakers will show a decline of 3.9 percent from a year ago, to 1.52 million vehicles.

This is "due to a quirk in the calendar" that pushes results of Labor Day weekend sales into September, JD Power and LMC said. While Labor Day is always in September, automakers normally count that weekend's sales in August results.

The annualized rate of sales for August will be 17.2 million vehicles, and the two consultancies maintained the forecast for 2015 at 17.1 million vehicles.

(Reporting by Bernie Woodall; Editing by Matthew Lewis)