HOUSTON  As the price of gasoline quadrupled over the last decade, American drivers seemed to defy the laws of economics by pumping more into their vehicles year after year.

But this is the year American drivers appear to be finally succumbing to price shock at the pump, according to a new report by Cambridge Energy Research Associates, a consulting firm affiliated with IHS Inc. It says the slowdown in the economy and soaring gasoline prices have finally persuaded Americans to drive fewer miles in fewer gas-guzzling vehicles.

“U.S. gasoline demand will likely decline in 2008 for the first time in more than 17 years,” says the report to be released Thursday. “For the first time since the 1970s and early 1980s the number of miles driven by Americans has clearly begun trending downward.”

The Transportation Department reported on Wednesday that Americans drove 1.8 percent fewer miles on public roads in April 2008 compared with the same month last year, the sixth consecutive month of driving mileage declines.