US economic growth shows new signs of weakening . But March consumer prices may overstate inflation

At first glance, the latest reports on the United States economy appear to herald a return to the days of rising inflation rates and slower growth. But the economic outlook is not as bad as statistics released Tuesday would suggest, forecasters say.

Economists say a 0.5 percent jump in the consumer price index (CPI) for March probably overstates the underlying inflation rate. But a 2.3 percent drop in orders for big-ticket items in March -- the third decline in durable-goods orders in four months -- is being viewed as a sign that US economic growth continues to slow.

``Inflation is still pretty much under control,'' says Bernard Markstein III of Chase Econometrics.

The slowdown in orders for durable goods, however, ``is a very worrisome thing,'' says Dorothea Otte of the Georgia State University Forecasting Project.

She says the decline could signal a buildup in inventories of unsold goods, which would put upward pressure on unemployment.

In addition to pushing up the unemployment rate, an overall slowdown in economic growth boosts the size of the federal deficit by cutting tax receipts and by raising the amount the government spends for unemployment compensation and other social programs.

Without action to cut the deficit, some economists warn, interest rates will rise later in the year, further slowing growth in the already weakened economy.

White House spokesman Larry Speakes said that ``prompt, decisive action by Congress in reducing federal spending is essential if we are to maintain the expansion of the US economy.''

The economy's apparent slowdown does have a silver lining: It has allowed interest rates to decline. Rates at this week's auction of Treasury bills dropped to their lowest level in more than two years.

The average rate on new 91-day T-bills dropped to 7.62 percent. And the rate on federal funds -- the overnight loans banks make to each other -- has dropped below 8 percent, vs. a rate of 8.5 percent in recent weeks.

Analysts are divided on whether the slowing economy on its own has caused the drop in rates or whether a cooling economy has prompted the Federal Reserve Board to ease credit conditions in a bid to keep the economy from slipping into recession.

Either way, some analysts say the recent decline in market interest rates will soon trigger a decline in major banks' prime, or benchmark, lending rates. Falling US interest rates are expected to keep downward pressure on the value of the dollar on foreign-exchange markets. In the last six weeks, the dollar has lost roughly 13 percent of its value against other major currencies.

The most troubling aspect of the fall in durable-goods orders -- which involve items expected to last three or more years -- was the 6.9 percent drop in orders for nondefense capital goods. This category includes the equipment that businesses use to make the products they sell.

The decline in business buying is worrisome because if it persists, it could indicate that companies are trimming their investment plans for the rest of 1985. If corporate spending plans were slashed, it would knock out a key underpinning of the Reagan administration's forecast that economic growth will rebound later in 1985.

The 0.5 percent hike in consumer prices in March, after seasonal adjustment, was fueled by soaring gasoline prices. If March's rate of increase were to continue for 12 months, it would represent an inflation rate of 5.8 percent.

Oil prices dropped too much last winter and now are correcting this spring, says David Wyss of Data Resources Inc. But the upturn ``does not reflect a basic shortage of oil.''