DETROIT; THE UNDERDOG; WHY AREN'T MORE PEOPLE BUYING US-BUILT AUTOMOBILES? HERE'S WHAT SOME OF OUR READERS HAVE TO SAY

Why aren't more people buying made-by-Detroit cars these days?

''I don't want to test-drive any more cars for General Motors,'' sniffs Diane Halferty, chairwoman of the Seattle-based Consumers Against General Motors, a group of GM diesel-car owners who are unhappy with their purchases. The complaint could apply to Ford Motor Company, American Motors, or Chrysler.

Responding to a questionnaire in the spring auto-motive/car-care section of The Christian Science Monitor April 16, hundreds of readers attack the US auto industry for being unmindful of what people want in cars. The replies ran 10-to- 1 against Detroit. We asked readers to complete ''I bought a new 1982-model US car because. . . ,'' or ''I decided to keep my old clunker because. . . .''

Mr. and Mrs. Gordon S. Dunlap of Yreka, Calif., assert: ''Barnum was right.''

Betty Howell of Los Angeles sums up the feeling of many motorists who are either staying out of the new-car market entirely or buying a car made in Europe or Japan. She writes as follows.

''I've decided to hold on to my old clunker because:

''Interest rates are far too high.

''American cars lose their value faster than foreign cars.

''Everyone knows US cars are built to last three years.

''The body style changes every three years or so.

''The US automobile industry has used all of the above, except No. 1, to keep the public in debt.''

Pointing to the income-cost gap that exists today, not to mention high unemployment, the letter shows the persistent credibility gap between the American car manufacturers and the motorists who buy their products.

Besides a bad economy, high interest rates, and unceasing pressure from the imports, the response shows the considerable obstacles facing US carmakers trying to entice more car buyers into the showroom. And the showroom itself may be a bad experience for some motorists, although this is not by any means limited to the domestic carmakers.

Herman Billings, of Miami, sighs: ''I'm keeping my old clunker, despite the fact I'd like to buy a new car, but I just can't bring myself to trust those crooked salesmen at the dealerships.''

In defense, Roger Smith, GM chairman, says: ''I'm very encouraged by what the dealers are doing on their own to bring themselves back into this thing. We have a new training program for salesmen, and we've increased our service training. What we're trying to do is make it easier for the mechanic by putting more self-diagnostic equipment into the cars. We've been very successful in getting a car to tell you, 'hey, I'm sick.' ''

Frank Wright of the University of Illinois, Champaign, retorts: ''I have a 1978 Chevette. Let me list a few of its problems, and you will see why I hesitate to consider a new American car.'' Then he goes into point-by-point detail.

Clearly, the domestic carmakers are on the defensive, and still have a great way to go to convince US car buyers that what Detroit builds is what they want.

[GM car sales in mid-June fell 25 percent below the same time period a year ago; Ford sales were off 17 percent.]

One thread ran through all the responses to the questionnaire as most respondents chimed in unison:

* The new cars cost too much money.

* Older cars are often more comfortable because of the additional room.

* The quality of the new-model cars is still in doubt.

''Why aren't Americans buying new US cars?'' asks Bert R. Caruthers of Spokane, Wash. ''The answer is twofold: quality and price,'' he replies.

Mark Janes Jr. of Deerfield, Mass., writes: ''The American consumer, however patriotic, wants top quality for top dollar. Currently, the trust and evidence rest with the imports.''

Yet Detroit carmakers, with their white-collar and production staffs cut to the bone, are spending billions of dollars to scuttle the image of which readers complain. But by their own admission, they still have a long way to go.

To underscore his point, Chandler Crawford of St. Louis wrote:

''In a package mailed today you will have your answer to why I've decided to keep my old clunker. The part is from a 1980 Pontiac Catalina with 42,825 miles. I had intended to send 'my offering' to the president of General Motors.''

The package contained a split engine hose. His point: US-built cars aren't built to last. Of course, writing to the ''top'' doesn't always work either, as motorists too often find out.

William T. Kinnaugh of Pompton Plains, N.J., writing about his 1981 Cadillac Eldorado, says: ''I wrote to the president of General Motors and the chairman of the board and, from the response - which incidentally did not come from them but from the customer-relations department - gave the feeling that the company does not care.''

Obviously not everyone is a critic of Detroit by a long shot. Some are buying late-model used cars and new cars built in the US, even though the cars' marketplace performance is far below the trend line.

Karla Watts of Fort Lauderdale, Fla., says:

''At this point we are a new family paying bills. We did, however, buy a used Ford because:

''It was American-made and we wanted a comfortable big car. Too, the price was right.

''I do feel when the time is right we will buy a new American car.''

The US automobile industry says it is investing some $80 billion over a 10 -year period - $40 billion by GM alone - in an all-out effort to compete with the imports and convince motorists that ''made-in-Detroit'' is not bad. As a matter of fact, US carmakers have made significant strides in upgrading the quality of the cars they build even as the vehicles themselves become more and more complex.

The head of Volkswagen's quality-assurance program in the US - and VW has an image of building high-quality vehicles - agrees that the US automobile industry is well on its way to building better cars.

''The US industry is learning to use the space in a smaller car more successfully,'' declares Dr. Ernst F. Beuler of VW.

''Quality is job one,'' proclaims Ford Motor Company, which then goes on to insist that the 1982-model cars are 48 percent better in quality than the Ford-built cars of 1980. But what does that say about the '80-model cars?

Robert W. Decker, GM vice-president for quality and reliability, says, ''We feel strongly that quality and customer satisfaction are synonymous.'' Yet GM is being attacked by consumer groups - by Ralph Nader-inspired organizations and the Seattle-based Consumers Against General Motors, which calls for a ''specific procedure'' to address consumer complaints.

The stakes are high for Detroit if it is to return to a viable position when the world economy improves.

H. E. Kubitschek of Hinsdale, Ill., writes:

''I decided to keep my old clunker because it retains more quality after four years than many US cars after only one year.''

Eli Adams Jr. of Barrington, Ill., adds:

''I will keep my old clunker (until the wheels fall off) because of the high prices the US automobile manufacturers and dealers are asking for their product. However, when it does come time to purchase a new car, it will be one produced by a US manufacturer.

''I hope by that time the automobile makers will see fit to bring down prices to a realistic level.''

Many readers objected to a reference to their older-type cars as a clunkers (said with tongue in cheek). Indeed, it shows the pride of ownership they feel in having a dependable car even though the odometer may read 100,000 miles or more.

''It's paid for,'' many of them say, ''and I don't want to impress anyone.''

Cynthia Johnson of Rochester, N.Y., asserts of her car:

''She's not an old clunker. As the advertising blurb goes: 'You're not getting older; you're getting better.' ''

Meanwhile, here is how more US car buyers see their vehicles:

Robert Norman King of Tiburon, Calif.

''Why don't they give us good mag wheels? Why do they use such cheap trim? Chrome or aluminum is not needed.''

Catherine M. Schifer of Brooklyn, N.Y.

''My old clunker is a 1966 4-door hardtop Pontiac Tempest. It has style and a big trunk, plus head room and leg room sufficient for six adults.''

Charles D. Parr of West End, N.J.

''If the '84 US cars offer value, I expect to buy. If not, then it will be an import for me.''

H. E. Kubitschek of Hinsdale, Ill.

''I decided to keep my old clunker because it retains more quality after four years than many US cars after only one year.''

B. Raymond Fink of Seattle

''While I will not go so far as to say I will definitely buy a foreign car next time, I'll certainly never buy another American car to order. The one I buy will be off the showroom floor instead of out of a catalogue. That way I'll know exactly what I'm getting.''

Mrs. W. E. Allan of Howey, Fla.''

You have hit a raw nerve. When I phoned agencies asking about base sticker prices so that I could make comparisons, I was told they would not discuss price on the phone.

''American dealers don't want to sell cars; that's why Americans aren't buying.''

Mr. and Mrs. George Shaw of Torrance, Calif.

''We are not looking for status symbols. We want reliable cars which are economical to buy, operate, and maintain.''

Joseph P. Herlihy of Sanford, Maine

''I prefer American cars because of the availability of service in this relatively rural community and the better sizes for my large family.''

B. J. Anderson of Tahoe City, Calif.

''Hasn't the car-buying public made clear its concerns over cost and efficiency by buying foreign.''

Harold Schipporeit of Salem, Ore.

''After driving and riding in foreign cars of friends of equal or higher price, we think our American car is superior in every respect, and most of our friends agree.''

Burt K. Filer of South Yarmouth, Mass.

''If you can afford a new car, you can afford an American car.''

Patricia Kupense of Easton, Conn.

''Quality more than price would probably be the deciding factor in buying a new car in the future.''

Earle Macartney of Palm Desert, Calif.

''Have you opened the hood of your automobile, no matter how tiny or large, and seen the collection of 'snakes' coiled up in there?''

Barbara Alexander of Greenfield, Mass.

''How I wish Detroit would make a plain, comfortable, reliable car without a lot of extras. Maybe we don't need so many choices and maybe we do need to cut the cost.''