The Trump Administration is expected to reveal more details about its revised vehicle fuel economy rules in upcoming days or weeks.

These rules could still include what the EPA and DOT proposed last year—a coordinated attempt to strip California (and thus all the states that observe its rules) of its ability to regulate vehicle greenhouse-gas emissions (GHG) on its own.

In remarks Thursday at a meeting between the California Air Resources Board and the California Transportation Commission, CARB chair Mary Nichols was threatening to get creative with some measures that could affect automakers and motorists even more—like an outright ban on vehicles with a tailpipe.

“I do have to say that the consequences are dire, as we’ve heard, and the alternatives that we face are extreme,” said Nichols, in a transcript of her closing statement provided by the agency. “If we don’t have the ability to continue to move forward with our transformation to cleaner cars, we will be faced with a dramatic alternative of tighter sector controls on everything else.”

DON’T MISS: Which states follow California's emission and zero-emission vehicle rules?

Tensions have built between the agencies over the past year and especially the past several months, as the Trump Administration has suggested that it might attempt—in a first-ever move for the federal environmental authority—to deny California its waiver to regulate motor vehicle greenhouse-gas (GHG) emissions.

Exhaust emissions from tailpipe [photo: Simone Ramella, 2005, used under Creative Commons 2.0]

Exhaust emissions from tailpipe [photo: Simone Ramella, 2005, used under Creative Commons 2.0]

But California has a workaround. If the state is denied its GHG waiver as part of this process, it will still likely maintain a waiver (decoupled from the coordinated package it was granted in 2013) to regulate conventional tailpipe air pollutants that directly cause smog and health issues—sometimes called criteria emissions—like nitrogen oxides, carbon monoxide, and sulfur dioxide.

Nichols hinted that this could include the movement of vehicles, or fees, taxes, and bans on certain types of vehicles and products. “And these are not things that most of us think are the right way to go,” she added.

In earlier draft remarks obtained by Bloomberg, Nichols had been due to note some even more specific examples, like health-protective regulations on refineries, doubling down on enforcement efforts for mobile and stationary sources, or an outright ban on internal combustion engines.

CHECK OUT: California, allies ready for emission-law war with Trump EPA, CARB head says

California’s Air Resources Board predates the EPA and maintains the authority to set tougher emissions standards.

Last August, the EPA and NHTSA jointly announced that they would seek capping vehicle fuel-efficiency at 2020 levels—a fleet-adjusted 37 mpg, versus the Obama Administration’s graduated 47-mpg target by 2025.

NHTSA's corporate average fuel economy (CAFE) expectations, via UMTRI

NHTSA's corporate average fuel economy (CAFE) expectations, via UMTRI

That led to a period in which industry lobbyists (and some automakers) pushed for a single national standard. But by February it was clear those talks had failed, and the White House, in a joint statement with the EPA and Transportation Department, said that CARB “had failed to put forward a productive alternative.”

READ MORE: Which states follow California's emission and zero-emission vehicle rules?

CARB meanwhile maintains that talks between the two agencies never got to the depth of discussing or negotiating policy.

The California agency hasn’t actually seen the final rule that the Trump Administration has put together, and while there is no firm plan at this point CARB “has begun evaluating possible options,” confirmed spokesman Dave Clegern to Green Car Reports.

If those options included any such ban on vehicles with internal combustion engines, even in a few smog-prone metro locations, it would be the first such restriction in the U.S. A number of international cities have taken action to restrict or ban internal combustion engines in particular zones. Amsterdam, for example, announced earlier this month that it would ban gasoline or diesel-fueled passenger vehicles (including motorcycles) by 2030, including pre-2005 diesel vehicles next year.