GM and Fiat Chrysler have reported that they purchased federal greenhouse gas emissions credits from Tesla, according to filings made to the state of Delaware and viewed by Bloomberg.
Both the US federal government and California offer automaker credits for selling zero-emissions vehicles. Those automakers can then sell those credits to automakers who exceed their pollution restrictions in a sort of cap-and-trade system that imposes extra cost on automakers not improving the fuel efficiency of their fleet.
Tesla has reported sales of its credits for years, but generally the buyers have been kept private. An exception to this occurred in April, when the Financial Times discovered a deal between Fiat Chrysler and Tesla that was reportedly worth hundreds of millions of euros. According to FT, Tesla had allowed Fiat to pool its European fleet with Tesla's in order to meet strict EU average fuel-economy emissions laws.