Car maker Mitsubishi Motors says "improper conduct" resulted in 625,000 of its vehicles getting inflated gas mileage ratings, in a scandal that's centered on mini-cars made for Japan's market.

The cars in question are Mitsubishi's eK Wagon and eK Space, as well as the Nissan Dayz and Dayz Roox (which the industrial giant made for Nissan Motors). While the scandal seems to be limited to the Japanese domestic market, Mitsubishi says it's now investigating vehicles it made for overseas markets, as well.

The fuel consumption cheat was uncovered by an internal inquiry that was sparked by Nissan's questions about irregularities in the cars' fuel consumption data.

Announcing its findings Wednesday, Mitsubishi said its employees manipulated the tires of cars that were being tested to provide official fuel statistics by improperly reducing the tires' running resistance.

"Following the announcement, shares in Mitsubishi tumbled," John Matthews tells our Newscast unit from Tokyo, "closing down more than 13 percent at the end of Tokyo trading."

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