DETROIT - General Motors will produce for the 2009 model year special fuel economy models of four of its biggest, thirstiest trucks, the Chevrolet Silverado and GMC Sierra pickups and the Chevy Tahoe and GMC Yukon SUVs.

Grouped under the moniker XFE, which Chevrolet has already used on a high-mpg version of the Cobalt small car, the trucks will return 15 mpg in the city and 21 on the highway. That represents a single mile-per-gallon improvement for both city and highway, compared to non-XFE versions with the same 5.3-liter V8 engine.

Unlike the two-mode hybrid versions of the company's big trucks, the XFE models use totally conventional means to achieve their improvements. All XFEs will use six-speed transmissions and a 3.08 axle ratio, and the Silverado and Sierra XFEs get aluminum lower control arms for the front suspension, and an aluminum spare wheel and aluminum 17-inch wheels wearing low-rolling-resistance tires (with higher recommended tire pressure). To improve aerodynamics, the pickups' suspension has been lowered and the XFEs carry deeper front airdams and soft tonneau covers.

The SUVs get the same mechanical changes as the pickups.

The XFE suite of equipment is offered only on two-wheel-drive versions of the trucks and SUVs and only in pickups in the crew-cab body style.

GM has not announced pricing on the models. They will be in dealership showrooms in the fourth quarter of this year.