How much additional CO2 emissions will result from recent changes to US vehicle regulations?

Question

From the Guardian, March 31, 2020: Trump to roll back Obama-era clean car rules in huge blow to climate fight (emphasis added):

The Trump administration is rolling back the US government’s strongest attempt to combat the climate crisis, weakening rules which compel auto companies to produce more fuel-efficient vehicles. [...] The changes to Obama-era regulations will allow vehicles to emit about a billion more tons of heat-trapping carbon dioxide – equivalent to roughly a fifth of annual US emissions.

Some are claiming that this change will directly result in the full billion tons of added emissions. But just because the requirement has changed, does not mean that automakers will automatically change the mileage or emissions controls on the vehicles they are producing. This is confirmed later in the article (emphasis added):

The Obama administration required auto companies to make vehicles 4.7% more efficient each year. The Trump administration initially wanted to freeze any progress on fuel efficiency past 2020. But its final rule, written by the EPA and the Department of Transportation, sets an improvement rate of 1.5% per year – or an industry average of 40.4 miles per gallon by 2026. That’s far less than the 2.4% per year by which the industry has said it will increase standards without any regulation.

Is there data or estimates on what the actual increase in emissions will be as a result of the new rule? Apparently the auto industry is capable of exceeding the current standard with no regulatory pressure, but will it now move the opposite way?

Answer 1

I'm not sure if this report was available at the time I asked the question, but in the process of approving the rule change, the Environmental Protection Agency prepared a regulatory impact analysis (pdf) that includes this estimate (emphasis added):

Today’s final standards are projected to increase CO₂ emissions compared to the previously issued standards, by a total of 867 million metric tons (MMT) over the lifetime of MY 1977 through MY 2029 vehicles [...] —i.e., by 2.9% of the amount projected to be attributable to passenger cars and light trucks under the baseline/augural standards.

Of this CO₂ emissions increase, 731 MMT would come from tailpipe emissions, and an additional 136 MMT from upstream sources, both being nearly 3% greater than projected to occur under the baseline/augural standards.

This table provides an estimated impact by model year:

While the rule itself only covers five years (2021 to 2026), other model years (MY) are included for a few reasons:

• Changes in new vehicle technology affect the resale value of older cars (MY 1977 to 2016). In this case, it is assumed that a lower efficiency standard means new cars are cheaper, increasing the rate at which old cars are taken off the road (scrapped), thereby reducing emissions for those model years.
• Vehicle technology changes are anticipated to persist for up to three years, meaning that changes implemented for MY 2026 will also affect MYs 2027 through 2029.