Montgomery County's economy is showing the first serious signs of slowdown, county officials said yesterday, and it could result in cutbacks in new police cars and a hiring freeze.

The cause for concern among county officials are preliminary revenue figures from the first quarter that could mean a shortfall of $30 million in the fiscal 1991 budget, county budget officials said. The projected shortfall is mainly in real estate and income taxes.

News of the projected shortfall prompted Montgomery officials this week to postpone spending requests of $1.3 million for 66 new take-home police cars and $400,000 for additional library books and video equipment.

Other contingency measures, including a "possible freeze on hiring," may be ordered if the shortfall grows worse, said County Executive Sidney Kramer. The county's total operating budget for fiscal 1991, which began July 1, is $1.6 billion.

Kramer pointed out that the county has a $42 million surplus fund, and said he is confident "Montgomery can ride out the tide."

"Until I have definitive numbers, I'm not pushing the panic button," Kramer said. "I will not be making any firm decisions until firm numbers are before me."

Montgomery is the latest jurisdiction in the Washington region to feel the pinch of the national economic downturn.

Prince George's County officials said Monday that they would trim hiring and limit spending after revenue projections indicated a $25 million shortfall. Earlier this month, Gov. William Donald Schaefer, citing an anticipated $150 million reduction in expected revenue, instituted a statewide hiring freeze and a cutback on vehicle purchases and government travel.

Virginia officials, meanwhile, are scrambling to slash $1.4 billion from the state's biennial budget.

The softening of the regional economy comes at time when Montgomery has evolved from a bedroom community of federal employees into a more diverse employment center, said Robert K. Kendal, director of the county's Office of Management and Budget. County officials are unsure whether the diversity will help cushion the economic hard times, he said.

In response to the anticipated shortfall, Kendal said his office is working on the "strictest budget we have seen in a number of years." Collection of state income tax since July is 40 percent below projections, according to county reports. Real estate transfer taxes are expected to be 19 percent below projections, officials said.

Kramer said top county finance officials plan to meet with state officials next week in Annapolis "to get a handle on the {revenue} numbers."

Officials said Montgomery's coveted AAA bond rating is not in jeopardy. "We will not leave the next government with a deficit," Kramer said.

Neal Potter, the 20-year County Council member who upset Kramer in the recent Democratic primary for county executive, said he is worried about the adequacy of the county's "rainy day" fund. "We are only left with a $42 million margin," he said. "That could be wiped out."

Potter, who called the revenue reductions serious, said if he wins the November general election, he might consider a temporary hiring freeze and the adoption of a wide range of user fees and new taxes on development, vehicle registration and gas.

The county's economic downturn could be exacerbated by the passage of one of four proposals on the November ballot to limit taxes and spending, Kramer said.

But Potter said he "could live with" a council-passed property tax question because it includes an escape clause for emergency situations.

Robert Denny, chairman of the taxpayer group Fairness in Taxation, said the county could save millions of dollars by eliminating "galloping waste" and duplication.

The County Council voted 4 to 3 Tuesday to defer for three weeks supplemental budget requests for the additional police cars and library materials.

Kramer had agreed to buy the police cars after the police union threatened a vote of no-confidence against Police Chief Donald E. Brooks. Yesterday, Walter Bader, president of Lodge 35 of the Fraternal Order of Police, said the council's action hurts recruitment. "It's another broken promise that reflects on their priorities," he said.