Well, now we know the numbers. And there are a lot of them.

The U.S. Environmental Protection Agency and Department of Transportation jointly proposed cutting greenhouse-gas emissions from medium and heavy duty trucks by 10 to 20 percent, starting in 2014 and extending through 2018. (The actual numbers vary greatly depending on the class of truck, of which there are many.)

That equates to raising the gas mileage for those vehicles by the same percentage. And because those trucks' fuel efficiency is so low to start with (anywhere from 4 to 10 miles per gallon), proponents say that even a 10- or 15-percent improvement could save enough money to cover the extra cost of necessary new technology.

10 to 20 percent, plus AC

For combination tractors, the propsed rules reduce carbon dioxide (CO2) emissions and fuel consumption up 20 percent from 2014 to 2018.

Heavy-duty pickups and vans will have separate standards depending on their engines; gasoline engines will emit up to 10 percent less, diesels up to 15 percent less. An additional 2 percent for each will come from reducing the leakage of greenhouse gases from those vehicles' air-conditioning systems.

And finally, for the huge category of heavy trucks known as "vocational vehicles" (special-purpose trucks for specific industries), the reductions will be up to 10 percent by 2018

The emissions rules measure not grams per mile of carbon emitted, as for passenger cars, but grams per ton-mile of carbon--reflecting an attempt to apply a single measurement across a huge variety of vehicle types. Similarly, fuel economy is not measured in miles per gallon but in consumption, using gallons per ton-mile.

Ray LaHood and President Barack Obama

Ray LaHood and President Barack Obama

U.S. Secretary of Transportation Ray LaHood and U.S. Environmental Protection Agency Administrator Lisa Jackson held a press conference this afternoon at which they announced the proposal. It applies only to on-road vehicles, meaning that mining trucks and similar specialized equipment is not covered.

'Win-win-win': $70 billion could be saved

Margo Oge, director of the EPA's Office of Transportation and Air Quality, told the Detroit News last week that the rules would be "historic," saying they would "advance energy security, climate change and will be a win-win situation."

Secretary LaHood added a third "win," saying that the rules would reduce reliance on oil, strengthen the country's energy security, and help mitigate climate change.

Last May, the EPA estimated the savings at 500 million barrels of oil over the lifetime of the vehicles, representing $70 billion in foregone diesel fuel costs. Net benefits to truckers, LaHood said, could be as high as $35 billion after the increased costs of higher-efficiency trucks.

Engines, tires, aero, and idling

Those costs would represent smaller, more fuel-efficient engines, lower rolling resistance tires, aerodynamic improvements, and a reduction in engine idling when vehicles are stopped. The agencies consider all of those "currently available" technologies, without radical new engineering required to implement them.

A reduction in idling, especially among certain types of semis and sleeper trucks that may idle up to 7 hours a day, could have huge paybacks. In the case of tractor-trailers, senior agency officials said, incremental costs of $6,000 or less could produce a lifetime payback of as much as $74,000.

The rules have been a long time coming, and they represent the first time that fuel economy requirements have been levied on the medium and heavy duty truck segments, whose carbon emissions represent approximately 20 percent of the transportation sector's total.

Tough Tractor Trailer

Tough Tractor Trailer

Hybrid buses and trucks too

While much media focus has gone toward electric and hybrid cars, many manufacturers--Eaton and Azure Dynamics, to name only two--are developing alternative drivetrains that improve the fuel efficiency of heavy vehicles. Most are hybrids, though electric delivery vans offer a payback on daily urban routes of  perhaps 80 miles or less.

In urban transit buses, hybrid powertrains can raise gas mileage from 4 to almost 6 miles per gallon. That may not sound impressive--because the MPG scale is confusing--but it's an improvement of 40 percent, which means huge fuel savings. New York, among other cities, now has several hundred hybrid buses in daily service.

Administrator Jackson said that many of the new rules would pay back the additional costs within a single year, an attractive proposition to fleet operators who base their purchase decisions on a vehicle's lifetime costs, not just purchase prices.

The proposal, which is not yet a formal rule, covers a huge variety of vehicle types, everything from heavy delivery vans to school buses, garbage trucks to 18-wheel long-haul semis. Medium duty trucks weigh 13,000 to 30,000 pounds, with heavy duty trucks coming in above that.

The rules apply to some heavy-duty versions of popular pickup trucks, including he Ford F-Series, Chevrolet Silverado and GMC Sierra, and Dodge Ram.

EPA & DoT pair up under presidential pressure

Because emissions are an EPA matter, while the DoT's National Highway Traffic Safety Administration is the agency that sets fuel-economy standards, the two agencies are working together for a single set of national standards.

That partnership was mandated by newly-elected president Obama earlier in his administration, and the pair jointly announced emissions reductions and gas-mileage increases for 2012-2016 passenger vehicles this past April.

The truck rules ensued, and the DoT and EPA are now considering what level of emissions reductions to propose for light-duty passenger vehicles in 2017 and later.

Heavy-truck drivers had urged that fuel economy for their vehicles remain with the NHTSA, which is required to take economic impact into consideration when setting rules, while the EPA is not. Legislation had been proposed--although not enacted--that would transfer the gas-mileage rulemaking from the NHTSA to the EPA.