The rumors started last week, and they were largely true: Buick plans to offer a model of its Regal compact sports sedan with the eAssist mild-hybrid system.

What we didn't have were the projected EPA mileage ratings (based on GM's internal tests). The company expects the 2012 Buick Regal with eAssist to be rated at 26 mpg city, 37 mpg highway, making it the most economical Buick model for 2012.

Unlike the larger 2012 Buick LaCrosse, which will fit the eAssist system as standard on the 2.4-liter Ecotec engine, the 2012 Regal will offer a base 2.4-liter engine without the system, and a version of the same engine with the fuel-saving powertrain as well.

The eAssist system consists of a small 0.5-kilowatt-hour lithium-ion battery pack, mounted at the rear of the Regal's trunk along with its power electronics and control circuitry. While there is a slight reduction in trunk space, the Regal fitted with eAssist maintains a pass-through from the trunk to the rear passenger compartment.

An engine-mounted electric motor replaces a standard alternator, and is connected with a special tensioned belt that allows it both to be driven by the engine and to restart it and contribute torque for short periods.

The alternator-start acts as a generator to recapture energy that would otherwise be lost as heat from the brakes, using it to recharge the battery pack. It can also act as an electric motor to start the engine, as well as contributing some torque to assist the engine and avoid the transmission shifting to a lower gear under load.

The system is tuned to provide high power rather than sustained energy delivery. It does not provide full electric running, but allows the engine to switch off as the car comes to a stop, and then switches it on again as the driver's foot begins to lift off the brake pedal.

While GM executives would not explicitly discuss projections for how many eAssist-equipped vehicles they hope to sell, they did acknowledge that the company could produce up to 100,000 eAssist-equipped vehicles per year if market demand reached that level.

That 100,000 number had been mentioned two years ago by GM executive Tom Stephens when the system (then known as Belt-Alternator-Starter II) was first described at the 2009 Geneva Motor Show.

Production of the 2012 Buick Regal with eAssist will begin in the fourth quarter of this year. No pricing for the eAssist option has been announced.