Elon Musk. REUTERS/Kevin Lamarque Tesla CEO Elon Musk fired back at a group of investors who are calling on the automaker to diversify its board.

Five investment groups penned a letter Monday asking for more independent board members who don't have personal ties to Musk to "provide a critical check on possible dysfunctional group dynamics." Musk's brother Kimbal sits on the board as does Brad Buss, the former chief financial officer of SolarCity, which Tesla acquired last November.

Musk responded to a Reuters article about the letter on Twitter, saying he already had stated his intention to bring on more independent board members during the merger with SolarCity. He also made a jab at Ford.

"This investor group should buy Ford stock. Their governance is amazing..." Musk tweeted.

Tesla recently surpassed Ford and General Motors in market cap, making it the most valuable US automaker.

The California-based company is currently preparing to roll out its mass-market Model 3 electric sedan and has an ambitious goal of selling 500,000 cars a year in 2018.