U.S. auto sales slumped to a 16-year low in July as automakers failed to keep up with consumers’ growing demand for smaller, more fuel-efficient vehicles. While production changes may help that problem, trouble in the credit and auto leasing markets will continue to take a toll on sales. General Motors, Ford, Toyota and other automakers said Friday that their U.S. sales fell by double-digits. Nissan Motor Co. was the only major automaker to report a gain, with truck sales up 18 percent thanks in part to the new Rogue crossover and a boost in incentives. Nissan’s overall sales rose 8.5 percent. Automakers were expecting a slide in July as high gas prices continued to cut into sales of trucks and sport utility vehicles and new troubles in the auto leasing sector further wrecked consumers’ confidence. July’s seasonally adjusted sales rate — which shows what sales would be if they continued at the same pace for the full year — was 12.5 million vehicles, according to Autodata Corp. That’s down from 17 million as recently as 2005. Automakers expect things to get worse before they get better. “We expect the second half of 2008 will be more challenging that the first half as economic and credit conditions weaken,” Ford’s marketing chief, Jim Farley, said in a statement. Mark LaNeve, GM’s vice president of North American sales, said tightening standards for buyers with poor credit are costing the automaker sales of about 10,000 vehicles pe ...