IRVINE, Calif. and SANTA MONICA, Calif. -

As September nears, both TrueCar and Kelley Blue Book are predicting new-car sales declines this month, while the number of used cars retailed is expected to grow.

According to TrueCar, total used sales, including franchised and independent dealerships and private-party transactions, may exceed 3,353,742, which would mark a 3.8-percent year-over-year increase.

The story is a bit different on the new side of the business.

KBB is predicting new-vehicle sales to drop by 4 percent year-over-year this month with a total of 1.52 million units sold resulting in an estimated seasonally adjusted annual rate of 17.2 million.

Over at TrueCar, the company is forecasting total new sales, including fleet deliveries, might decrease by 2.9 percent year-over-year, but it is predicting the SAAR should reach 17.4 million units this month versus 17.3 a year ago — even with a slight decline in volume.

According to both reports, the week Labor Day falls on this year is playing a role in expected declines.

Last year, auto sales from Labor Day weekend were included in August results, while this year, they will be parceled into September 2015 sales. Also, there are 26 sales days in August 2015, compared to 27 sales days in August 2014.  

“Despite the tough comparison with last August, robust demand for crossovers and pickup trucks continues this month and the industry is right on plan to hit our revised 17.2 million-unit projection for 2015,” said Eric Lyman, TrueCar’s vice president of industry insights. “Last August was a very strong month, with a built-in sales bump from Labor Day. With the Labor Day delay, we anticipate a lot of buying activity in September.”

According to KBB, due to the exclusion of Labor Day in August sales results, the company expects most automakers to report sales decline in August — with one exception.

KBB analysts explained that with Ford’s F-150 growing both in sales and available inventory, the automaker could see an increase in sales in August, following a “slow launch” earlier this year.

And TrueCar reported Kia Motors America as the expected sales leader among major automakers with a forecasts increase in new sales of 5.2 percent.

According to the KBB report, retail sales are forecasted to make up 88.8 percent of new-car sales volume in August, up from 88.2 percent last year.

Alec Gutierrez, senior analyst for Kelley Blue Book, also pointed out that in looking at predicated August results, one must look outside the U.S.

"While the outlook for August remains bright, we must keep an eye on the financial markets which have declined precipitously in the last few weeks on uncertainty in international markets, namely China," he said.  "We remain confident that sales in August will remain robust; however, should the U.S. financial markets continue to falter, we could see demand for new cars soften in the short to medium term.  It should be noted that the unemployment rate in the U.S. remains below 6 percent, while the auto finance environment remains as attractive as ever, so we don't necessarily expect to see the sales pace deviate from its current 17 million-plus SAAR trajectory for 2015 unless the stock market continues its downward trajectory in the weeks and months to come."

As for what segments will see the most sales this month, KBB anticipate the compact utility segment will continue to see sales soar while the car segment market share might suffer a bit.

KBB expects compact SUV/crossover new sales will reach 225,000 this month for a year-over-year increase of 7.7 percent. On the other hand, the mid-size car segments is predicted to see a 9.4 percent decline from last year, while compact car sales will likely decline by 7.5 percent.

"Kelley Blue Book anticipates that the compact utility segment will top all others in August.  With a host of new models at price points around $20,000, and with gas prices remaining reasonable, this segment has never been more popular," said Gutierrez.  "As a result of moderate gas prices and increased interest in utility vehicles, we expect market share for cars to fall once again in August.  Year-to-date, sales of cars comprise 44.5 percent of the market, whereas they made up 47.3 percent of sales last year."