The original comment period was set to end June 13. The extension has moved the public comment deadline to July 1.
The EPA extended the comment period following complaints from critics that the agency did not give enough time for adequate public input.
Following the extension, industry group American Petroleum Institute has argued that the formal comment period should last 90 days.
If passed, the Tier 3 rulemaking would set new vehicle emissions standards and lower the sulfur content of gasoline in 2017.