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Hong Kong-listed shares of China's top electric carmaker, BYD Co. Ltd. (BYDDF)  , have risen 46% in the last six days on investor confidence that the country will soon set forth goals to eliminate vehicles powered by fossil fuels. But shareholders could be jumping the gun, according to a Bloomberg report.

BYD's relative strength index reached 85 Wednesday, Sept. 20, which is higher than any of its fellow Hang Seng China Enterprises Index peers. Relative strength index readings above 70 are typically signs a stock is overbought, according to Bloomberg.

This could be cause for concern, as investors appear to be overly optimistic that China will soon establish a formal timetable for eliminating vehicles that run on fossil fuels and open the electric vehicle industry to more foreign investment. China's lawmakers back the move, but many analysts think any plans will likely be long-term goals that won't boost earnings soon.

BYD, which is backed by billionaire investor Warren Buffett, leads the booming market for electric vehicles in China. During 2016, Chinese drivers registered 352,000 electric vehicles, compared to only 159,000 in the U.S. during the same period.

Hong Kong-listed BYD shares increased as much as 18% Wednesday before finishing trading with a gain of 12.7%. Shares have soared nearly 70% since the start of 2017. BYD's over-the-counter shares trading in U.S. markets also rose sharply, gaining 12% in the last five days.

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