WASHINGTON — Will your new car get an A in fuel efficiency? A government proposal may add letter grades to showroom window stickers on new cars and trucks to reflect a vehicle’s overall fuel economy and greenhouse gas emissions.

The Transportation Department and the Environmental Protection Agency said Monday they are considering two options to upgrade the energy and environmental information that will adorn labels on new vehicles in car dealership showrooms, beginning with the 2012 model year.

The government is considering a letter grade approach or updating the design of the current sticker to include comparisons of a vehicle’s fuel economy and tailpipe emissions.

Consumers scan the window stickers to compare vehicles when shopping for a new car or truck. The stickers have not been updated significantly in three decades, and the government wants the labels to reflect emerging technologies and account for greenhouse gas emissions affecting the environment.

“From electric to plug-in hybrid vehicles, we think a new label is absolutely necessary to help consumers make the right decision for their wallet and for the environment,” said Gina McCarthy, the EPA’s top air pollution official. The changes are required under a 2007 energy law.

Under the letter grade proposal, vehicles would be ranked on fuel efficiency and emissions. The letter grade would also include an estimate of how much money a motorist would save in fuel costs over five years.

Automakers questioned the proposed letter grades, saying they might affect sales. Alliance of Automobile Manufacturers spokeswoman Gloria Bergquist said “the letter grade inadvertently suggests a value judgment.” She said a broad range of vehicle technologies were needed to improve fuel efficiency.

McCarthy said the letter grade option was not meant to be a judgment on the vehicle, but a “metric that consumers can use” when car shopping.

The second option would maintain the current label’s focus on a vehicle’s miles per gallon rating and annual fuel costs, but it would update the design and add new comparison information on fuel efficiency and vehicle tailpipe emissions.

David Strickland, head of the National Highway Traffic Safety Administration, said there was “no preferred option,” and the government hoped to hear from the public during a 60-day comment period. The public can e-mail comments on the plans to newlabels@epa.gov. A final plan is expected in early 2011.