Until she was 25, Deanna Sclar was one of the "dummies" she writes about.

She didn't drive and knew nothing about the insides of an automobile. It didn't matter; she lived in Brooklyn and took the subway to get around.

When she and her husband moved to Southern California -- where much of life is spent on a freeway -- she bought a used car and started commuting 30 miles a day each way to a job for a publishing firm.

At that point she began to worry. What would happen if something went wrong with her 6-year-old Mustang on one of those high-speed commutes? She'd be helpless.

A friend talked her into going to an auto-repair class, and that changed her life.

"I was like a duck finding a pond," she says. Once she began to learn about what makes autos run, she fell in love with them. It's an affair that is still blossoming.

Sclar is so smitten that she will tell you outright that besides saving a bundle of money doing some of your own maintenance, you can "have a hell of a good time working on your car."

The fun comes, she says, "because it's so easy, and you expected to hate it, or be bored or befuddled."

Sclar, who earned a degree in comparative literature at Brandeis and Harvard universities, wrote a book, "Auto Repair for Dummies" (McGraw Hill, 351 pages, $7.95) that has sold about 175,000 copies since 1976. It is now finding its way from bookstore stacks to auto-supply sales counters.

She is parttime consumer consultant for Uniroyal tires, traveling about the country to demonstrate a new tire designed to automatically seal small punctures in the tread. In September, she drove from Boston to Atlanta with up to 20 three-inch-long nails pounded into the tires to show the seal's effectiveness.

And, to her delight, she writes a regular auto-repair column for the Explorer Scout magazine aimed at the 16-to 22-year-old car-crazy male. She's also trying her hand at a screenplay -- "an adventure story."

She gave up her Mustang for a 1959 Mercedes roadster "which I restored myself. It took me four years." She and her husband also own those two highway lovers' dream machines, a pickup and a van. She's in her 30s, but won't say exactly where, though she has a daughter 17 and a son 13.

The message in Sclar's book is that anybody can do the routine work on their car and save a lot of money, as well as assuring themselves that the job is getting done properly.

Once, she said, a lady in Beverly Hills "with tons of jewelry" told her she had bought her book and was going to change the oil in her own car. "I must have looked incredulous," Sclar says, because the woman remarked immediately:

"You know, rich people don't like to get ripped off either."

Sclar says an oil change is a simple process that may cost you as little as $6.50, including a new oil filter if you do it yourself. "If you take the car into the shop, it could run from $15 to $20."

The savings isn't in money alone. It may take an hour or more to drop the car off at a garage, but you should be able to do it at home in 10 minutes. "And 9 of those 10 minutes is just waiting for the oil to drain."

What do you do with those five quarts of used oil? Sclar collects it in a plastic bag placed in a basin. She either takes it to a friendly service station that will dispose of it, or ties it up and puts it in the trash barrel. s

"Changing the oil regularly," she says "is the secret to having a car live forever." She recommends a change every 2,000 to 3,000 miles or every two or three months. At that frequency, the do-it-yourself savings pile up.

Then, she says, there's that personal satisfaction when you change the oil filter and realize "I can control this monster. It's great fun."

Sclar says she doesn't want to knock auto mechanics, but it is not unknown for a mechanic doing an ignition tuneup to change all of the spark plugs except one that he may find particularly difficult and time-consuming to reach. e

"You can just forget it if they're not all changed. It's like going to a symphony orchestra and one guy is playing "Melancholy Baby.'"

The savings can be big, too. A tuneup at the gargage could run from $50 to $300. You might be able to do it yourself for as little as $10, she says, although the outlay the first time would be a bit more if you have to buy tools.

Most of the tools you should already have at home, she says, but if you must buy, don't go for the best. "You're not a mechanic and you're not going to use them eight hours a day."

She warns that the first time you try your own tuneup, it may take longer. Nuts may have been tightened with mechanical wrenches "or grown into place" if the car is an old one. She does a tuneup in a half-hour.

The worst thing that can happen the first time is that when you are all finished "the car won't start." Her advice: "Just go back over everything and find out what you did wrong. It's not going to blow up."

Even if you don't want to get your hands dirty attempting to cure whatever ails your vehicle, it still pays to be famliar with what makes a car work and what might go wrong, she says, so you're not at the complete mercy of the service shop. She believes the mechanic might even be more sympathetic if he senses you share some of his love for cars.

"I do not believe mechanics are ripoff artists, but we sure tempt them."

When she must take her car to the shop Sclar tries to schedule work at the end of the day when the mechanics are more relaxed. "I usually bring a six-pack of beer and I pump their brains unmercifully."