President Reagan said today that a 5-cent-a-gallon increase in the federal gasoline tax will cost the average car owner only about $30 a year, "less than the cost of a couple of shock absorbers."

In his weekly radio address broadcast from Rancho del Cielo, his mountain ranch near here, the president made his case for increasing the gasoline tax from the current 4 cents a gallon in a program to repair the nation's crumbling transportation system.

The president hailed "freedom of travel and the romance of the road" as "vital parts of our heritage," but added, "Let's face it. Lately, driving isn't as much fun as it used to be."

"Time and wear have taken their toll on America's roads and highways. In some places the bad conditions of the pavement does more to control speed than the speed limit," he said.

Earlier this week, he endorsed the 5-cent boost, and Congress is expected to approve the program in the lame-duck session beginning Monday. While congressional leaders have described the program's design explicitly as creating jobs as well as fixing roads and bridges, the president has insisted that it is not an effective jobs program.

Today, however, he took a slightly different tack: "The program will . . . stimulate 170,000 jobs, not in make-work projects, but in real worthwhile work in the hard-hit construction industries, and an additional 150,000 jobs in related industries."

Although others have described it as a straightforward tax increase, the president said it will not "add to the taxes that you and I pay on April 15," referring to personal income tax. "It will be paid for by those others who use the system, and it will cost the average car owner only about $30 a year."

"Most important of all, it will cost far less to act now than it would to delay until further damage is done," Reagan said.

Offering some examples of the deteriorating conditions of the nation's highways, Reagan said, "There's already a section of interstate highway in Illinois that's so far gone the truckers call it the only all-gravel interstate in the country."

He said Little League baseball had to be suspended on a field underneath the Queensboro Bridge in New York City because of the bridge's condition. "And in one small community in Pennsylvania, a family living beneath a bridge that's part of I-70 has told their children not to play outdoors because rubble from the bridge keeps falling into the yard," he added.

Reagan said that 4,000 miles of interstate highway need resurfacing and that 23,000 bridges need replacement or repair. He noted that the gasoline tax has not been increased in 23 years.

He said his goal is to preserve for future generations "a highway system that has long been the envy of the world and that has truly made the average American driver king of the road."

In the Democratic response, focused largely on Reagan's speech last Monday about nuclear arms, Sen. Paul E. Tsongas (Mass.) said the gasoline tax increase had bipartisan support "long before the president signed on." He urged congressional enactment "as rapidly as possible."