In 15 years, a new car that gets less than 50 miles per gallon could be considered a gas-guzzler--if new fuel economy regulations President Obama plans to announce tomorrow stick. Automakers have agreed to support the new standards, which would U.S. vehicles to average 54.5 miles per gallon by 2025. Current standards require an average fuel economy of 31.4 miles per gallon by 2016.

"If upheld, the plan would lead to the biggest gains in fuel economy since government began setting mileage regulations in the 1970s and could lead to substantial changes to the cars and trucks most Americans drive," according to the Wall Street Journal.

The impact of the fuel economy standards will depend on the details of the regulations. Previous fuel economy regulations have made exceptions for light trucks and SUVs, which helped lead to a boom in sales of these vehicles, cancelling out much of the reduction in fuel consumption provided by more efficient cars.