The Obama administration has argued that the stricter mileage rules will save drivers about $1.7 trillion in fuel costs and reduce oil consumption by 12 billion barrels through 2025.
Lawmakers already used a package of tax loopholes and fee increases to provide the cover than $10 billion shortfall in gas tax revenue in the current transportation bill, which is set to expire in 2014.
Advocates and observers say Congress will likely have to consider other revenue sources for transportation funding, with or without higher mileage standards. At least one idea that is often floated among dedicated transportation supporters, a tax on miles traveled by a vehicle (VMT), has been disavowed by both the Obama administration and Republicans in Congress.
The GOP included an anti-VMT plank in the platform it approved during its convention this week in Tampa, Fla.
"We oppose any funding mechanism that would involve governmental monitoring of every car and truck in the nation," the GOP platform says of the VMT proposal.