THE Federal Reserve can lead bank customers to lower interest rates, but it can't make them borrow.That's the reaction of many economists and businessmen to the Fed's latest cut in its discount rate, which aims to promote spending in order to accelerate the pace of recovery from the 1990-91 recession. The rate cut is "a positive step that improves the consumer's ability to finance new cars. The uncertainty is what kind of effect that will have on the consumer's willingness," says Terry Sullivan, a spokesman for General Motors Corporation, which along with the auto industry as a whole has been starved for sales. "This will help, but it's not going to be anything dramatic," adds Richard Peterson, chief economist at Continental Bank in Chicago. Concern over the economy is making consumers cautious about spending, he says. The discount rate is the interest rate that the Fed charges member banks for loans. Adjustments to the rate are a strong signal of monetary policy: downward to stimulate economic growth; upward to arrest inflation. Banks generally adjust their own interest rates accordingly. On Wednesday the Fed set the discount rate at 4.5 percent. That was the fifth rate reduction since Dec. 17, when it was 7 percent, and the lowest it's been since 1973. A number of major banks quickly announced similar cuts. The initial effect of lower interest rates is smaller repayments on floating-rate debt. The effect is staggered because interest rates on corporate borrowings, for instance, might be recalculated every month while the rates on some mortgages are adjusted only once a year. "For [every] person who's paying less interest there's [a] person who's receiving less interest," notes Steven Strongin, an economist with the Chicago Fed. But the economy still gets a boost because a decline in interest income doesn't curtail spending plans as much as a decline in interest costs stimulates spending by making debt more affordable. Individual debtors will use the savings to pay off debts faster, bank it, or take it shopping. "That's exactly what the Fed is trying to engineer here," says Wayne Ayers, chief economist at the Bank of Boston. "Even debt reduction improves net worth and over time that puts the consumer in a better position to spend." Businesses are likely to bank their own interest rate savings until the extra dollars from consumers begin flowing in. Then they will consider spending to expand operations. "Before firms add to their labor force, they've got to see a sustained pickup in demand for their product," Mr. Ayers says. "Employment always lags." Diane Swonk, an economist with First National Bank of Chicago, sees the increase in demand for bank loans coming in the first months of 1992. But banks won't meet that demand in every case. "On speculative lending to people who want to build an office building, you betcha, there is a huge credit crunch," she says. "We will not fund." "You'd have to be a fool to think that an office building going up right now is going to produce revenues enough to pay off the loan," Ms. Swonk adds. The national vacancy rate for downtown office space exceeds 17 percent, and tops 20 percent in the suburbs. Rents are running at cost, she says. Mr. Sullivan says a rebound in auto sales won't spark new plant construction. Capacity is a million units above North America's normal vehicle demand of 15 million to 16 million units. Demand next year will be 2 million units below normal, he expects. But while new plants are out of the question until at least 2000, Sullivan says, a sales rebound would prompt auto companies to engage in all kinds of discretionary spending, from advertising to materials purchases, that would help the economy. "We're on thin ice right now," Swonk says. "The economy in the fourth quarter will look like it's coming almost to a standstill." With elections coming next year, "the pressure is mounting" for an improvement to the economy. She expects the Fed to reduce the discount rate again by up to half a percentage point, possibly before Christmas, even though it can't yet see the impact from the previous rate cut in September. The timing of this week's rate cut was political, asserts Michael Cosgrove, a professor at the University of Dallas who writes the Econoclast newsletter. Otherwise, he says, the Fed would have waited until retail sales data are released in two weeks. He agrees that the state of the economy warranted the rate reduction, but he says that what's really needed is a growth-oriented tax policy, namely elimination of the capital-gains tax. Inflation has been running at 3.5 percent, excluding fuel and food prices. "By late 1992, after the election is over, the Fed is going to have to start worrying again about how much stimulus they put into the economy and inflation," Swonk says.