Consumers are enjoying the greatest gains in purchasing power in at least three years as a result of plunging oil prices, sliding interest rates, the robust stock market and increases in their incomes.

Consumer prices have dropped in the past three months at the steepest rate since 1954; interest rates are back to levels of eight years ago; the price of oil is half its level of a few months ago, and the stock market has reached record highs, providing consumers with more money in their pockets and on paper.

But economists are divided on what consumers will do with these new-found gains, and consequently how much of a boost it will give economic activity.

Consumer spending is important because it accounts for about two-thirds of gross national product -- the nation's total output of goods and services.

Traditionally, consumers spend about 95 cents of every new dollar gained, economists say, and lately they have been spending closer to 97 cents. Demographics also suggest that spending will increase, as the disproportionately large age group between 25 and 44 enjoys the prime time of their lives for buying homes, furniture, appliances and cars, economists said.

After-tax income adjusted for inflation in the last four months has risen at the sharpest rate in two years, which should increase consumption in the months ahead, said Robert Wescott, managing director of U.S. services for Wharton Econometrics. Higher incomes generally lead to more spending.

Westcott expects consumption to pick up briskly in the third and fourth quarters from the current quarter "because people are finally going to realize when prices are lower . . . they really have stronger real income than they thought and they're going to spend it."

But offsetting the positive trends are the historically high debt levels of consumers and low savings rates, suggesting that some consumers may salt away some of their new wealth or pay off bills. Or they could shift more of their purchases from U.S.-made goods to imports, limiting the boost to domestic economic activity, economists said.

With so many possibilities, it is no surprise that economists disagree on how this higher purchasing power will impact the economy, with some suggesting it could increase GNP by as much as 1 percentage point this year while others say it will cause hardly a ripple.

"None of those forces in and of themselves amount to a row of pins," said Steven Malin, senior economist for The Conference Board, commenting on the influence of lower interest rates and oil costs and higher stock prices.

"I think these windfall gains mean it will be less likely there will be a collapse in consumer spending," said Allen Sinai, chief economist for Shearson Lehman Bros. "In real terms, the lower oil and gasoline prices will produce more growth in real consumption than might have been otherwise expected. That will help keep real GNP going at a reasonably good clip: 3.5 percent fourth quarter to fourth quarter."

Wescott said these gains, along with an improvement in the trade picture so that U.S. producers get a larger share of domestic sales, will translate into GNP growth of 4.3 percent in the third quarter and 5.5 percent in the fourth.

The decline in the price of oil is expected to have broad impact on the economy, but some economists are skeptical about how savings of, say, $10 a week on gasoline purchases will influence a consumer's spending patterns. For the most part, people will just drive more, particularly on vacations, these economists reason. Moreover, the threat of terrorism is sure to cut down on overseas travel and put more cars on U.S. highways this summer, some analysts said.

Economist Alan Greenspan expects average auto mileage in this country to increase from 9,800 in 1985 to 10,300 in 1987, and sees national gasoline consumption increasing from about 4.8 million barrels a day in 1985 to 5.2 million barrels a day next year.

Although the volume of gasoline purchases will increase, the dollar amount will fall, economists said. And the benefits of lower oil prices will be felt by other energy products as well.

"We have consumers spending about $93 billion at an annual rate on gasoline in the fourth quarter," said Kurt Brown, a Data Resources Inc. economist. "Nominal spending by the fourth quarter of '86 will be about $77 billion. That will give us about $16 billion in just lower gasoline prices.

"If we look at fuel oil, there's probably a saving of about $4 billion" for home heating oil, Brown said. "Electricity, may save $1 billion or $2 billion , natural gas about $3 billion."

DRI has predicted that for every $1-a-barrel drop in the price of oil, American households will save $35 a year, or a total of $3.8 billion. This year, prices have declined from $27 a barrel to $14.

Brown said consumers will save indirectly from lower fuel costs for airlines, which should stimulate competititon and help keep ticket prices down. Furthermore, producers may be able to hold the price line on goods that use petroleum products, such as many plastics, Brown said.

The impact of lower gasoline costs "wouldn't make a big dent in big-ticket items, but it would impact on services and nondurables," Sinai said.

Spending on nondurables -- goods expected to last less than three years -- increased in March, Sinai said. Spending on services was up "considerably," he said.

"It's not enough to make someone buy a car," Sinai said of gasoline savings. "It's pocket change that would be used and spent freely for small items -- extra lunch out, entertainment, recreation, leisure."

Oil price savings also vary widely by region. For example, lower gasoline prices have a bigger impact on consumers in the West than on those in the Northeast, where there is greater reliance on mass transit, said Malin of the Conference Board.

Consumers in the Northeast will get bigger savings in fuel-oil costs, Malin said. However, in the Midwest "where the heating source of choice has been natural gas, there is not quite the same kicker" as with heating oil. Natural gas has become relatively more expensive, Malin said.

The biggest impact from the decline in interest rates could be felt in the housing market, economists said, as homeowners with heavy mortgages refinance their homes at lower rates and reduce their monthly payments by $100 or more. Some economists don't see refinancings leading to a big surge of spending for consumer goods. But first-time home buyers and those trading up to larger houses can be expected to buy furniture, appliances and other items.

Lower rates are not expected to help auto sales much because the major auto makers have been offering their own discount financing plans, economists said.

Another potential result of lower rates cited by Sinai is a boom in spending for home improvements.

However, if a new surge of buying for consumer goods is directed laregly at imports, it would cut down on domestic growth, Sinai and others said. "If you're buying a VCR, the odds are 95 to 100 percent it's going to be a foreign buy," Sinai said.

The benefits of a bullish stock market are another matter entirely, economists said. Since many investors do not sell their stocks to cash in, there is no immediate impact on spending. But, the economists added, investors may spend more because they feel richer..

Higher stock prices "raise confidence very strongly," said Robert Ortner, the Commerce Department's chief economist. "It's the psychological impact of seeing reports on TV night after night, and stock owners know what their holdings are worth. I have found in the past there are strong correlations between stock market prices and various indexes of consumer sentiment."