Suzuki Motor used the wrong methods to test the fuel economy of its cars in Japan, it said on Wednesday, widening a testing scandal that has already rocked smaller rival Mitsubishi Motors Corp.
Japan's transport ministry ordered widespread checks to industry methods after Mitsubishi Motors admitted last month it manipulated fuel economy data for at least four mini-vehicle models, overstating their efficiency.
Mitsubishi Motors' president, Tetsuro Aikawa, said on Wednesday he would step down over that scandal, becoming the first senior departure since it broke, battering the company's reputation and wiping billions off its market value.
Worries over similar damage for Suzuki sent shares in Japan's fourth-largest carmaker down as much as 15 percent during the day.
Suzuki said it would continue selling its mini-cars and saw no impact on its earnings. The correct readings, it said, were not significantly different to those it submitted.
Japanese authorities, however, have asked for further details from Suzuki before May 31, calling its use of non-compliant tests "outrageous."
"The company apologizes for the fact that we did not follow rules set by the country," CEO Osamu Suzuki told reporters, adding 2.1 million vehicles were affected.
The company denied using the tests to make its fuel economy data look better. It blamed the decision to cobble together readings from individual car parts - rather than a single reading - on its windy testing location on a coastal hill that made readings erratic. The individual parts were tested indoors, it said.
Suzuki specializes in mini-vehicles, which have engines of up to 660cc and get preferential tax treatment under Japanese law. It has roughly a third of the country's mini-vehicle market. It also has a successful Indian subsidiary, Maruti Suzuki.