TOKYO—Toyota Motor Corp. has retained its post as the world's best-selling auto maker, saying it sold 10.15 million vehicles world-wide in 2015.

The company also said Wednesday it was considering taking full control of small-car maker Daihatsu Motor Co., in which it currently holds a 51.2% stake. Toyota and Daihatsu said they were looking at that option and others in response to a report in the Nikkei newspaper.

Toyota's 2015 group sales, which include subsidiaries Daihatsu and Hino Motors Ltd., fell 0.8% from 2014, the car maker said Wednesday. They declined slightly year-over-year because strong sales in the U.S. couldn't offset slowdowns in Southeast Asia and other emerging markets.

The Japanese giant has kept a sales edge over General Motors Co. and Volkswagen AG with its relatively balanced product portfolio and global reach, though the race remains close. Toyota has kept the No. 1 spot for seven out of the last eight years after besting GM in 2008, falling out of the top spot only in 2011 when it struggled to resume production after an earthquake struck Japan.

Toyota, Volkswagen and GM account for about a third of the vehicles sold world-wide. Around 90 million vehicles were sold world-wide in 2015, according to estimates by IHS Automotive. In 2015, Volkswagen sold 9.9 million vehicles, but struggled late in the year after a scandal involving falsified emissions erupted in September. GM sold 9.8 million vehicles, benefiting from strong demand in the U.S. and growth in China.

Toyota has been considering cooperating with Suzuki Motor Co. over fuel-saving and safety technologies and over operations in India, a Suzuki stronghold, a person knowledgeable about the matter said. The Nikkei reported earlier that Toyota and Suzuki were in partnership negotiations, and both companies denied that they have started such talks.

Shares in Suzuki, Japan's fourth-biggest auto maker by global sales volume, rose more than 10% in early trading. Daihatsu shares were untraded. Toyota shares were up about 3% in early afternoon trade.

Toyota already has capital ties or business partnerships with several of Japan's smaller car makers, including Subaru parent Fuji Heavy Industries Ltd., Mazda Motor Corp. and truck maker Isuzu Motors Ltd.

Write to Yoko Kubota at yoko.kubota@wsj.com

(END) Dow Jones Newswires

January 27, 2016 00:35 ET (05:35 GMT)