Ford Motor Co. spent five years and some $3 billion developing a lighter-weight version of its F-150 pickup that burns less fuel. And that’s the problem: Gasoline prices have plummeted 15% in the last four months as it geared up to sell the new model. A technical tour de force, the new aluminum-bodied truck was designed and conceived for an era of higher fuel prices. It weighs 700 pounds less, is as much as 20% more fuel-efficient, and can tow up to 11% more weight than its predecessor. Due to go on sale in December, the 2015 model represents a bet by Ford that it can sell customers on new technologies to meet future fuel-economy standards. But an oil glut has undermined the company’s gamble on a vehicle that contributes 50% of its earnings in North America.