"The single thing that can start the wheels of industry turning again is further reduction of interest rates." President Reagan in the State of the Union message.

Since July, the interest that consumers pay to borrow money for cars and houses has dropped markedly, bringing new life to these two depressed industries.

But most other rates--such as those on credit cards and personal loans--remain nearly as high as they were during the summer.

Many economists are worried that, unless consumer borrowing costs fall more than they have, retail sales and other key elements of the economy will be slow to recover from the recession.

Whether that decline in rates will occur this year is anything but certain. On Thursday the influential Wall Street economist Henry Kaufman predicted that short-term interest rates will continue to decline, but long-term rates--which are of more concern to consumer lenders--will show unprecedented volatility in 1983. This could tend to keep consumer rates higher than might otherwise be expected.

The prime lending rate for corporate borrowers has fallen from 16 1/2 percent on July 1 to 11 percent at most banks today. The federal funds rate, which is the interest that banks charge each other for overnight loans, has fallen from about 14.8 percent to 8 1/2 percent during the same period.

New-car loans at banks have fallen nearly as fast, from about 17 percent to 13 1/2 to 14 percent in the same period (although they are as high as 16 percent in Chicago). But banks are making few car loans because auto companies themselves are offering 11.9 percent financing to lure buyers to showrooms.

However, rates on credit cards have been rising, rather than falling, and most other consumer rates remain in the 17- to-19-percent range at banks and as high as 30 percent or more at finance companies.

Ben Laden, chief economist for the investment firm T. Rowe Price Associates Inc., said that, unless consumer rates come down further--say to the point where banks also charge 12 percent for automobile loans--consumer borrowing costs will "continue to act as a significant drag on the economy" and delay the long-hoped-for economic recovery.

Pollsters at the University of Michigan Survey Research Center said high rates and continuing worries about the economy will keep consumers on the sidelines for some time to come. Their surveys support Laden. Director Richard Curtin said half the respondents to a poll last fall said they would start buying big-ticket items such as cars and appliances when interest rates fall to 12 percent.

Many economists and lenders say they are not sure how much consumer rates will come down. Not only are most consumer loans costlier to make than business loans, but the cost of consumer deposits is rising. Banks generally use their consumer deposits to "fund" consumer laons.

Except for home mortgage loans--where the amount borrowed usually is sizable--changes in consumer borrowing rates have little effect on the consumer's monthly payment--what most borrowers and lenders look at when deciding whether a loan is affordable.

But economists and lenders say interest rates play a strong psychological role in consumer buying decisions, especially in a period of economic uncertainty. Experts say, for example, the aggressive promotion of lower new-car financing charges by major automobile companies seems to have stimulated a big surge in car buying.

Last summer, the going rate for an auto loan was about 18 percent. On Oct. 1 General Motors Corp. began to offer 12.8 percent financing. GM, Ford Motor Co. and Chrysler Corp. all offered 10.9 percent rates from Nov. 1 to Dec. 31 and began offering 11.9 percent financing the first of this year.

Automobile sales rose steadily from an annual rate of about 4.7 million vehicles in early October to 6.5 million in mid-January.

The lower rates themselves do not mean a lot to the average customer's monthly payments. According to Marc Goloven, vice president and financial economist at Manufacturers Hanover Trust Co., for the average new-car loan of about $7,400, a decline of 1 percentage point in financing costs translates into a $4 decline in the monthly payment. For smaller loans, the monthly impact is even less.

Many lenders say interest rates are not as important as consumer confidence.

"People need to see some stability in the economy. Interest rates play a part, but other things are far more critical," according to Charles Wheatley, vice president for consumer lending at Maryland National Bank.

The one area where the interest rate plays a real as well as psycholgical role is the home mortgage loan, where a reduction of one percentage point in the interest rate will cut monthly payments by between $50 and $60.

A six-percentage-point drop might save a car buyer $25 a month or less. It would save a home buyer $300 or more.

Last July the average rate on a 30-year home loan was 16.71 percent at savings and loan associations. Today the rate is between 12 1/2 and 13 percent. During the same period, sales of new and existing family homes jumped from an annual rate of 2.24 million to 2.75 million, according to figures supplied by the Commerce Department and the National Association of Realtors.

This week, however, American Security Bank raised its mortgage rate from 12 7/8 percent to 13 3/8 percent. A spokesman said the bank ties its mortgage rate to a number of other interest charges--such as those determined at auctions of Treasury bills and Federal National Mortgage Association mortgages. He said that those rates have risen recently and that, and as a result, the bank has boosted its charge.