What kind of gas mileage do your trucks get? If you buy a replacement rig starting in 2014, you could find yourself behind the wheel of a vehicle that–by government mandate–will be up to 15% more fuel-efficient and spew fewer greenhouse gases to boot.

That's the intent of a proposal made recently by two federal agencies to create the first national greenhouse gas and fuel efficiency standards for heavy-duty trucks and buses. The standards would affect everything from monster pickup trucks (gross vehicle weight above 8,500 pounds) to the biggest tractor-trailers.

Under one proposal, the standards would be voluntary for the 2014 and 2015 model years and then become mandatory on a phased-in basis from the 2016 through the 2018 or 2019 model years.

Washington estimates the program would generate $41 billion in net benefits over the lifetimes of the vehicles produced between 2014 and 2018. For instance, it said the fuel efficiency gains in a semi-trailer truck would pay for the increased costs of technology in under a year, and yield as much as $74,000 over the truck's useful life.

The changes are expected to increase the price of a new vehicle, though it's unclear by how much prices would rise. Then again, most dealers contacted by ProSales in recent years haven't been buying any brand-new trucks since the housing crash. Most who have bought trucks got them used from dealers that went out of business.