The Environmental Protection Agency and the National Highway Traffic Safety Administration unveiled their Safer Affordable Fuel-Efficient Vehicles Rule back in August, but a number of states vowed to fight the proposal which aims to freeze fuel economy standards at 2020 levels.

The California Air Resources Board (CARB) has followed through on its threat and voted to keep the original, Obama era standards in place.

This means automakers will need to have a corporate average fuel economy rating of 54.5 mpg (65.4 mpg UK / 4.3L/100km) by 2025. In comparison, the federal government is only aiming for an average of 37 mpg (44 mpg UK / 6.3L/100km). That’s a massive difference of 17.5 mpg (21 mpg UK / 13.4L/100km), or roughly the equivalent of what a Lamborghini Aventador (supposedly…) gets on the highway.

CARB’s decision means that California and 12 other states could have their own fuel economy standards. The original plan, which was adopted in 2012 under President Obama, would have created nationwide standards.

The decision will have a huge impact on automakers as the 13 states are responsible for more a third of the light passenger vehicles that sold in America. In effect, it would likely push many companies to develop more fuel-efficient vehicles than the federal government requires.

Of course, the Trump administration hasn’t been shy about threatening to revoke California’s ability to set its own fuel economy standards. Back in April, then EPA Administrator Scott Pruitt said: “Cooperative federalism doesn’t mean that one state can dictate standards for the rest of the country.” The government also said “The California waiver is still being reexamined by EPA under Administrator Pruitt’s leadership.”

The decision likely sets the stage for another court battle, but CARB Chair Mary D. Nichols said: “Today’s vote by the Board ensures that California and 12 other states will not fall victim to the Trump administration’s rollback of vehicle standards should its proposal be finalized.”