Ford Motor Co. is extending its July Fourth incentives through the summer and General Motors Corp. is doubling the incentives on its new full-size pickups in an attempt to boost slumping sales, reported The Detroit News.

Ford's offers include zero-percent financing for 36 months on all 2007 Ford, Lincoln and Mercury models, as well as $2,007 in cash on trucks and SUVs. Customers can opt for up to $2,500 cash instead of the free financing.

Sales of Ford's domestic brands were down 8 percent last month and are down 11.3 percent for the year.

One bright spot in June was truck sales, which rose 2.6 percent — a sharp contrast to the double-digit declines in truck demand reported by the other U.S. automakers. Ford F-Series outsold its top rival, the Chevy Silverado, by more than 20,000 units in June.

To maintain momentum, Ford is offering dealers that meet July F-Series sales goals a chance to sell one of 50 Shelby GT500 KR editions when they go on sale next spring. Ford plans to produce 1,000 of these 540-horsepower sports cars.

GM is trying to recover lost ground by offering up to $2,000 in cash on the Silverado and its sibling, the GMC Sierra. The automaker is also offering to pay up to six months' lease payments for owners of its midsize SUVs, as well as the Chevy Equinox and Pontiac Torrent, who trade up to a 2007 Buick Ranier, GMC Envoy or Chevy Trailblazer and finance through GMAC.