A group of Tesla Inc investors has urged the luxury electric car maker to add two new independent directors to its board, without ties to Chief Executive Elon Musk, to "provide a critical check on possible dysfunctional group dynamics."

A defiant Musk took to Twitter on Wednesday, April 12 afternoon to suggest the investors buy stock in Ford Motor Co instead. The Ford family controls the Detroit automaker through two classes of stock, Reuters reports.

In a letter dated Monday, five investment groups including the California State Teachers Retirement System, Hermes Equity Ownership Services and CtW Investment Group urged Tesla to have all of its directors re-elected annually.

"We expect that as companies make the transition to publicly-traded status, the governance structures and practices in place at the time of the IPO will evolve to align with the company's changing strategy," the letter reads. "However, Tesla's seven-member board is largely unchanged from its pre-IPO days."

Led by the enigmatic Musk, Tesla recently became the most valuable U.S. car company, passing General Motors Co for the top spot.

Tesla's market value has since slipped to just shy of GM's. As of Wednesday, the market cap of the Silicon Valley automaker was $50.3 billion, while GM's was $50.8 billion.

"This investor group should buy Ford stock," Musk posted on Twitter on Wednesday afternoon. "Their governance is amazing..."

Musk then said on Twitter that he would follow up soon on an earlier promise to appoint more independent directors, "but this (investor) group has nothing to do with it."