Not all U.S. vehicles that burn gasoline or diesel fuel are passenger car and light trucks.
Medium- and heavy-duty commercial trucks make up a small percentage of the traffic on U.S. roads, but they cover more miles and have far worse fuel efficiency than passenger cars.
MORE: Is The BulletTruck What Semis Will Look Like In The Future?
That's why trucks are the focus of the latest environmental initiative by President Barack Obama.
Speaking Tuesday at a Safeway distribution center in Upper Marlboro, Maryland, Obama instructed Secretary of Transportation Anthony Foxx and Environmental Protection Agency Administrator Gina McCarthy to develop stricter fuel-economy standards for large trucks.
Cummins and Peterbilt 'SuperTruck'
Proposal by March 2015
He said the Department of Transportation and EPA will consult with truck manufacturers, large shippers, and other stakeholders to produce a set of proposed rules by March of next year.
The call for additional rules follows Obama's promise in his State of the Union Address to use executive power to implement his administration's policies, rather than rely on a combative Congress.
The new rules will follow fuel-economy standards for medium- and heavy-duty trucks passed in 2011.
Those regulations take effect this year, and call for a 10- to 20-percent increase in fuel economy by 2018, depending on the class of vehicle.
The next round of regulations will run from 2019 through 2025, bringing large trucks into line with the corporate average fuel economy (CAFE) rules for passenger vehicles, which now run through 2025 as well..
Small improvements, big savings
Such relatively small increments can have a large impact.
Trucks rack up far more miles than passenger vehicles--with far worse fuel economy--so they produce a disproportionate amount of emissions.
In his speech, Obama noted that heavy-duty trucks make up 4 percent of traffic on U.S. roads, but account for 20 percent of the carbon pollution from the transportation sector.
Even an improvement from 4 to 6 mpg could result in a savings of 8 gallons of fuel for every 100 miles traveled.
AirFlow BulletTruck (Images: AirFlow Truck Company)
Diesel industry on board
Thus far, makers of diesel engines appear optimistic that they will be able to produce more fuel-efficient trucks that comply with any new regulations.
The Diesel Technology Forum--which represents diesel-engine manufacturers--said in a statement that the industry has already made great improvements to diesel efficiency, and should be able to keep pace with the Obama Administration's plans.
Over the past decade, the group says, strict new emissions limits have cut emissions from the latest large-truck engines by 99 percent for nitrogen oxides and 98 percent for particulates.
Whether truckers themselves approve of the new measures remains to be seen; many long-haul truckers opposed transferring oversight of heavy-truck fuel efficiency from the DoT's National Highway Traffic Safety Administration (NHTSA) to the EPA back in 2010.
They asked the DOT to intervene and save them from the EPA, which had drafted the proposal for the regulations that take effect this year, because they feared it would raise the cost of new trucks.