The Volkswagen diesel scandal has led to increased scrutiny on all carmakers' compliance with emission and related fuel-economy rules.
Last month, Mitsubishi admitted to manipulating fuel-efficiency tests in Japan, resulting in overstated fuel-economy ratings for over 620,000 small "kei" cars.
Now Suzuki has followed with a confession of its own.
The company said it used the wrong methods to test fuel economy on Japanese-market models, Reuters reported Wednesday.
Suzuki officials said 2.1 million vehicles are affected, covering 16 models sold in Japan.
The company claims models sold outside its home market are unaffected.
Suzuki is Japan's fourth-largest automaker, but it hasn't sold cars in the U.S. since it withdrew from the market after the 2013 model year.
The admission came after the Japanese transport ministry ordered widespread checks of testing methods in response to the Mitsubishi scandal.
Suzuki said the correct fuel-economy ratings were not vastly different from the overstated ones it originally submitted.
But Japanese officials reportedly called the company's actions "outrageous," and demanded more information from Suzuki before May 31.
Suzuki shares also plunged 15 percent the day news of the fuel-economy cheating broke, although the company reportedly isn't concerned about a long-term negative impact on its earnings.
Like Mitsubishi, Suzuki's misreported fuel-economy results are related to methods for calculating air resistance.
Suzuki SX4 S-Cross
Fuel-economy tests are conducted in a laboratory setting with the car strapped to a device called a dynamometer, so air resistance must be calculated separately and factored in.
This is often done with a "coast-down test," where a car coasts to a stop from a given speed.
Mitsubishi admitted to tweaking the parameters of the coast-down test to achieve better results.
However Suzuki claims its tests were merely inaccurate.
It claims that rather than simply taking overall measurements, it cobbled together figures from measurements of individual components—includes the brakes, tires, and transmission—and data from wind-tunnel tests.
It also blamed typically-high winds at its Sagara proving ground in coastal Japan for skewing results.
The Japanese government will wait to receive additional information from Suzuki before it decides on penalties.
Officials haven't yet decided to penalize Mitsubishi for its indiscretions.
Last week, Mitsubishi received a lifeline from Nissan, which bought a 34-percent controlling stake in the smaller carmaker for $2.17 billion.
Japanese transport ministry officials said Wednesday that Suzuki and Mitsubishi are the only two companies that used improper fuel-economy testing methods.