Smaller cars of today need better maintenance

''My car runs just fine,'' one owner declares. ''It doesn't need a tuneup.''

Another chimes in: ''I'm pretty handy with a wrench; I can work on the car myself.''

Are they right?

Experienced mechanics agree that you do notm save money by neglecting your car or working on a machine with which you're unfamiliar. You could damage your car, or even drastically shorten its life, by failing to give it proper maintenance.

If you want to keep your car more than five years without running up expensive repair bills, you'd be wise to follow a regular preventive-maintenance schedule. There is major repair work you should not attempt, however, especially repairs associated with the emission and electronic engine controls.

''This is the area where the vehicle has become more sophisticated,'' says Burt Schimpke, manager of the advanced service engineering department at the Ford Motor Company. ''They should go to their dealer.''

At the same time, there are many other things that a motorist can do alone, such as changing a battery, brake pads, and muffler. A few do-it-yourselfers are even ambitious enough to overhaul the engine. Many gain the technical know-how through car-maintenance courses, help from automotive mechanics, and by using auto-maintenance guides.

Some people have a misconception about today's smaller cars and think they can drive them the way they drove big tankers of 10 or 20 years ago.

George Giek of the American Automobile Association (AAA) explains: ''The big old cast-iron V-8s didn't require as much maintenance as the new down-sized cars.''

''By necessity,'' he adds, ''the newer cars work harder and need more maintenance. It's an educational process that new-car buyers have to go through to learn that they can't abuse today's cars as they may have abused a car in the 1950s or '60s. For example, there is less oil in the crankcase, most of the engine parts are made out of the lighter aluminum, and the engines operate at a higher r.p.m.''

The AAA spokesman says car owners should say to themselves, ''The car is changing and I have to change my habits along with it.''

It's a motorist's responsibility to keep track of the maintenance schedule for his car. Study the owner's manual in the glove compartment.

If you decide to perform some of the preventive-maintenance jobs yourself, here are a few suggestions on what you can reasonably expect to do without having to get involved with the space-age components found in the late-model car:

* Check over the entire engine compartment and under the car for obvious signs of loose parts or fluid leaks. If you find something that looks suspicious , be sure to take care of it fast.

* Run the engine and test-drive the car, listening for any unusual sounds. Again, if something doesn't sound right, take the car to a mechanic. This is where communication is important.

Describe the sounds to the technician and under what circumstances you heard them, such as when the car is first started, when the engine is hot or cold, or when it runs at a certain speed. Make sure your mechanic has a written description of the problems and check over a repair order carefully before signing it.

* Weekly, and especially before long trips, check all the fluid levels (oil, transmission, windshield washer, brake, power steering, and battery if it's the serviceable type), as well as the air pressure in the tires. Make sure the alternator and other drive belts have no more than half an inch of ''give'' between the pulleys. Also, closely examine the radiator hoses, drive belts, and windshield-wiper blades for signs of wear and cracking.

* Change the engine oil and filter every 4,000 to 5,000 miles or four to five months, whichever comes first.

Some automobile manufacturers have extended the oil-change intervals because of improved lubricants and engineering. But these recommendations are under ideal driving conditions. City driving and frequent short trips reduce the interval significantly.

Have a mechanic show you how to change the oil properly. You can also use a car-maintenance guide for specific information. Under the car, the differential (if it's a rear-wheel drive) should be serviced and any fittings greased.

* Replace the air filter at least twice a year, and more often if you drive in dusty and wintry conditions. In addition to the main air filter, some cars have an air filter associated with the emissions-control system that can be easily replaced.

* If you have worked with spark plugs before, you can clean them, adjust the gap, or replace them every 12,000 miles. Electronic ignitions, which are found on most of today's domestic cars and on more than half of the new imports, dramatically reduce the wear on spark plugs, thus increasing the manufacturers' suggested change intervals to more than 20,000 miles.

''Cars cost a lot of money these days,'' notes Mr. Giek of the AAA. Don't waste it.