TOKYO—Suzuki Motor Corp. on Tuesday said it used an improper fuel economy testing method on more models than it had previously announced, the latest revelation in a scandal that has rocked the Japanese auto industry.

Suzuki said it had used a testing method that wasn't approved by Japanese regulators on 26 models, having previously said 16 were affected, but maintained it had no intention of improving mileage.

Results from tests using an approved method showed no significant fuel economy difference, Suzuki said.

Japan's transportation ministry had asked the company to submit a detailed report on the issue by Tuesday.

In measuring the resistance that a car faces from tires and air to calculate fuel economy, Suzuki had said previously that around 2.1 million vehicles sold in Japan were affected. On Tuesday it updated that figure to 2.14 million vehicles sold in the country.

Suzuki said that after further probing, it found that the correct testing method was used in three of the 16 models it previously mentioned, while an inappropriate testing method was used in one additional model.

It also said 12 other models supplied to other makers also used an inappropriate testing method. It didn't elaborate on what other auto makers were affected.

However, Suzuki said results from testing all models using the correct method showed better fuel economy data compared with what it has listed on its catalogs.

"We would like to do everything we can to prevent recurrences," Suzuki's Chief Executive Officer Osamu Suzuki said at a news conference.

The report comes after Mitsubishi Motors Corp. admitted to manipulating fuel-economy-related data and using an improper testing method for some models in Japan for 25 years. The company has said its president will step down to take responsibility.

Auto makers selling vehicles in Japan were earlier this month required to report to the nation's regulators whether they had violated mileage-related rules.

They are required to submit a range of fuel economy data to the transportation ministry, including data from on-road test driving. Suzuki conducted indoor testing. To prevent recurrences, it said it would renovate its on-road test driving course to comply with the regulation, while strengthening internal checking systems.

Mr. Suzuki said it was his duty to oversee the new measures and said he didn't intend to step down at this point.

Suzuki executives said the adoption of an improper testing method wasn't intentionally done to improve mileage. The company has said that no vehicles sold outside of Japan are affected.

Suzuki's report comes as global auto makers come under scrutiny over overstatement of mileage or cheating on emissions. The problem first came under light late last year after Volkswagen AG said it used illegal software on some of its diesel cars to cheat on U.S. emissions tests.

Write to Alexander Martin at alexander.martin@wsj.com

(END) Dow Jones Newswires

May 31, 2016 06:55 ET (10:55 GMT)