The company's US unit FCA US had previously celebrated a streak of monthly year-over-year sales gains going back to April 2010.
In revising its methodology on calculating the data, the company now says its streak ended in September 2013, with a year-over-year decline that month of three percent.
Sales also declined in August 2015 (one percent) and May 2016 (seven percent).
"Our review of industry practice has not revealed a standard reporting practice" for monthly sales, the company said in a statement.
It added that reporting in the US is different than other markets because companies are left to their own devices to estimate sales.
In all the revision affected 4,500 over-reported sales out of a total of 7.7 million reported in the period between January 2011 and June 2016, Fiat said.
FCA US said it will take more directly into account vehicles that dealers report as sold, but then reverse to count as still in stock.
"The objective of this new methodology is to provide in FCA US's judgment the best available estimate of the number of FCA US vehicles sold to end users through the end of a particular month," the company said.
Last week, the car maker pledged to cooperate with probes by the Securities and Exchange Commission and the Justice Department, which are reportedly examining whether the company violated securities laws.
FCA US maintained Tuesday that the previously inflated car sales figures did not impact its reported revenues in financial statements.
In January, a Fiat Chrysler dealers group sued the car maker, accusing it of financially rewarding dealerships that falsely reported higher car sales.
FCA strongly denied the allegations, dismissing the arguments as the product of "two disgruntled dealers" who failed to fulfill their commitments to FCA.
Fiat Chrysler shares were up 1.8 percent at €6.30 ($6.99) in afternoon trade.