General Motors is tweaking some of its full-size pickups and sport-utility vehicles, using lighter materials and sleeker aerodynamics in an effort to bolster their fuel economy.The Chevrolet Silverado and Tahoe and the GMC Sierra and Yukon are being packaged with the refinements. They get special badging, XFE, which stands for Xtra Fuel Economy. The upgrades allow the vehicles to eek out an extra mile per gallon in fuel ratings--they now get 15 mpg in the city and 21 mpg on the highway.The XFE package also includes six-speed automatic transmissions, which are an optional upgrade on other models from the four-speed gearboxes. The transmission change increases towing capacity to 7,000 pounds in the Silverado and Sierra pickups, though the SUVs' tow ratings remain unchanged. Other improvements include a lowered suspension, specially-tuned chassis, low-rolling-resistance tires and lighter-weight aluminum wheels. Production of the vehicles, which are 2009 models, begins this fall.