"This really constitutes a national security crisis in the making," said letter signer Frank Gaffney, head of the Center for Security Policy, a thinktank, and a former Defense Department official under former President Reagan.

Other signers included Robert McFarlane, Reagan's national security advisor, and James Woolsey, Central Intelligence Agency director under President Clinton.

In an uncharacteristic move, the security experts sought input from groups like the Natural Resources Defense Council, which have long lobbied for more fuel-efficient cars.

"It's strange bedfellows but this is actually the real American majority," said Nicole St. Clair, a spokeswoman for the NRDC. "It's common sense."

Policymakers should address rampant oil demand from gas-guzzling vehicles, and stop trying to solve the problem by opening land like the Arctic National Wildlife Refuge to drilling, she said.

The letter urged the government to encourage car makers to design vehicles from lighter materials to improve mileage. It also endorsed the use of "plug power" -- hybrid vehicles that can run off internal batteries for short trips before switching to their internal-combustion engines.

The program would cost $1 billion over five years.

Regulations known as Corporate Average Fuel Economy (CAFE) standards require automakers to achieve an average fuel economy of 27.5 miles per gallon for all passenger cars sold, and 20.7 mpg for vans, sport utility vehicles and pick-up trucks. The standards have not been tightened for more than a dozen years due to opposition from Detroit.

The average fuel economy has steadily dropped since 1988. It was 20.8 mpg for all 2003 model vehicles, according to the Environmental Protection Agency's annual mileage report.

McFarlane told the White House that stricter mileage standards could help cut US crude oil imports in half.

The group's recommendations gave short shrift to hydrogen-powered vehicles, a Bush administration priority, because they will take decades to field.

US drivers should not depend on foreign suppliers like Saudi Arabia for security reasons, they said. Although Saudi officials say the kingdom's oilfields are protected from terror attacks, McFarlane said the oil installations are "extremely vulnerable from a military point of view."