Consumer prices rose 0.4 percent in September, led by the largest jump in gasoline prices in 16 months, the Labor Department reported yesterday.

The inflation figure -- the last inflation statistic before the Nov. 6 election -- was considered moderate by most economists. The Reagan administration has made the slowdown in the inflation rate a major point in the president's reelection campaign.

Last month's price figures will have an impact on millions of taxpayers and Social Security recipients. The September price report will be used to calculate the cost of living allowance for Social Security payments as well as determining the new set of tax indexing rates, which go into effect in 1985.

Last month, the consumer price index stood at 314.5, which means that a basket of goods that cost $10 in 1967 last month cost $31.45. The CPI-W, the index used for adjusting Social Security payments, some other federal payments and some collective bargaining agreements, was 312.1.

Except for sharp rises in gasoline prices and the costs of apparel, prices remained under control nearly across the board. Food prices declined slightly in September.

The 1.1 percent seasonally adjusted increase in gasoline prices was the sharpest rise since prices rose 3.1 percent in May 1983, Labor reported.

Gasoline prices have declined 4.0 percent so far this year and are 14.5 percent below the peak reached in March 1981, Labor said.

Part of the increase last month was because of a cutback in production by U.S. oil companies to help reduce plentiful gasoline supplies, economists said.

As supplies tightened, retailers were able to raise prices somewhat, economists said.

However, because of recent weakness in prices of major oil-producing countries and the threatened breakup of the Organization of Petroleum Exporting Countries, economists said they don't expect any great upward pressure on oil prices, and that gasoline prices may decline by two cents to three cents a gallon in the coming weeks.

"Consumers can be confident that their paychecks remain safe from inflation," said White House spokesman Larry Speakes.

Steven Wood, an economist with Chase Econometrics, predicted, "We're not going to get any acceleration in inflation, not anytime in the next six months." Wood said monthly price increases will probably return to the lower rates experienced in the beginning of the year when prices rose only 0.2 percent or 0.3 percent each month.

For the first nine months this year, consumer prices rose at a seasonally adjusted annual rate of 4.2 percent. The annual rate for the third quarter was 4.5 percent, following increases of 3.3 percent in the second quarter and 5.0 percent in the first quarter, the Labor Department said.

For the 12 months ended in September, consumer prices rose 4.2 percent. However, that rate is far above the 2.4 percent increase reached in July last year, the lowest one-year rate since the recovery began.

Meanwhile, the Commerce Department said in a separate report that real gross weekly earnings of U.S. workers increased 0.5 percent in September. These earnings had declined 1.1 percent in August and 0.4 percent in July.

Gross weekly earnings were $299.27 last month compared with $294.65 in August.

Jerry Jasinowski, chief economist for the National Association of Manufacturers said that prices "are currently experiencing downward pressure" because rapid growth in the economy is slowing and commodity prices have declined.

He also said that the appreciation of the dollar against other major currencies is a major factor in the good price picture. A high-valued dollar makes imports relatively cheap, putting price pressure on domestic goods producers.

In addition, oil prices worldwide appear to be dropping, producing downward pressure on future oil prices.

"Under these conditions, there is a good chance that inflation for all of 1984 will be less than 4 percent," Jasinowski said.

However, other economists said they see inflation accelerating in some areas, such as apparel, home furnishings and new cars.

"The running inflation rate has picked up in the last few months," said John Maher, economist with Citicorp Information Systems. "As demand increased, businesses increased their profit margins. These prices are sticking."

Maher said he didn't expect furiously accelerating inflation in the next few months, but he said that the plateau of price increases has moved up to a higher level. Instead of monthly increases of 0.2 percent or 0.3 percent during the spring, increases will probably average about 0.4 percent for a while, Maher said.

Food and beverage prices, which rose 0.6 percent in August, dropped 0.1 percent last month. Grocery store food prices dropped 0.3 percent, reflecting declines in prices for meats, eggs and vegetables.

Dairy product prices rose 0.9 percent, the largest increase since the end of 1980, Labor said.

In the housing category, charges for gas and electricity, which both had risen sharply in July and August, declined 0.1 percent and increased 0.7 percent respectively. Fuel oil prices dropped 0.3 percent, the fourth consecutive monthly decline.

For the last three months apparel costs have risen at an 8.3 percent annual rate and housing expenditures have risen 6.5 percent, Labor said.