New fuel-economy rules for trucks to be announced this week would not only impose mileage standards for the first time on previously exempt heavy-duty models, such as the Hummer H2, but also would overhaul standards for other trucks, according to auto industry officials and others familiar with the plan.

They declined to be identified because the National Highway Traffic Safety Administration hadn’t made the new rules public.

The rules could pressure automakers to improve the mileage of their heftiest vehicles, often known as “three-quarter-ton” or “one-ton” trucks. And they could create, for automakers, a bewildering array of fuel-economy standards that could vary almost from model to model.

The revamped standards apply only to trucks. Car standards don’t change.

The NHTSA regulations determine whether an automaker is fined for selling too many low-mileage trucks, or not enough high-mileage trucks. They have nothing to do with mileage numbers printed on the price stickers of new vehicles.

The heart of the new system is an index number calculated using the wheelbase and track. Wheelbase is the length from the center of the front wheel to the center of the rear wheel. Track is a width measurement, based on how far apart left and right wheels are.

The most lenient requirements would be for the largest vehicles, those with long wheelbases and wide tracks. The strictest would apply to the smallest models.

The rules differ from a system proposed in August. It would have split trucks into six size categories and assigned standards to each category, from about 21 to 28 miles per gallon.

Environmentalists complained that automakers simply would make small changes in the sizes of their vehicles to get them into categories with less-strict mileage requirements.

Current regulations have a single truck standard, 21.6 miles per gallon for 2006 models. An automaker calculates the mileage of all trucks sold and, if it’s lower than 21.6, must pay a fine.

Heavy-duty pickups would continue to be exempt because they are considered work trucks rather than personal vehicles. That could amount to a significant loophole, according to the Union of Concerned Scientists, which calculates that 80% of heavy-duty trucks sold are pickups.

Federal officials wanted a new system that doesn’t encourage selling more small trucks in order to meet mileage standards. Statistics show that bigger, heavier vehicles are safer to their occupants in crashes than smaller, lighter vehicles are.

If the new standards encourage larger vehicles, “from a safety perspective, that’s good,” says Adrian Lund, president of the Insurance Institute for Highway Safety.