Reducing the monthly operating expenses of owning a car

''I have a two-year-old car,'' a motorist asserts. ''I've just realigned the front end and replaced the shocks, put in new steering and suspension parts, and replaced the wheel bearings. Still, I find my car-operating costs are up even though I'm getting good mileage on my compact.''

But are you really trying to keep your car costs down? All drivers can reduce monthly operating expenses if they follow some common-sense practices.

A car, for example, uses about 30 percent more gasoline at 70 miles an hour than at 50. But cutting down on fast driving is only one of the many sensible suggestions in reducing the cost of owning and operating a car.

Here are some others:

* Watch out for potholes. A pothole can cause severe damage, not only to the tires and wheel rims, but also to the front-end alignment and other crucial front-end structures on an auto.

The Tire Industry Safety Council (TISC) suggests that drivers maintain proper tire inflation and drive more slowly on chuckhole-infested roadways.

''Keep your eyes on the road and keep the wheels out of the holes if you want to keep your car out of the garage,'' the council advises. Remember, however, not to endanger other motorists as you avoid a pothole.

* Inflate tires properly. Underinflated tires do not roll smoothly, and they also decrease gas mileage.

Drivers can save more than $30 a year by taking care of the tires on their car. ''When fully inflated,'' the TISC says, ''tires are safer, because a lack of air can decrease tread life and reduce steering and car response.'' Also, you can lose about 2 percent in fuel economy for every pound of pressure under the tiremaker's recommended level.

Check tire air pressure when the tires are cold. Hot tires can register as much as 6 pounds over the actual pressure.

* Use retreads. Tire companies are now manufacturing quality retreads that are hard to distinguish from new tires.

Manufacturers replace new treads for worn-out treads on sound casings. The airline and trucking industries use retreads extensively.

* Baby the engine. Harmful carbon deposits build up from an idling engine and from traveling at low, stop-and-go speeds. Before the stress of speed, a cold engine needs a little time for oil that has settled in the crankcase to warm and flow.

Avoid stomping on the gas pedal and racing a cold engine after starting. Instead, idle the engine briefly, especially on cold days. Also, avoid racing the engine before turning it off. Racing causes raw gas to wash away protective oil film around the pistons. Instead, let the engine run at idle for a second or two before shut-off, especially after high speeds.

Have the engine serviced at the recommended appropriate intervals suggested in the owner's manual.

Incidentally, regular engine servicing can reduce auto pollutants by up to 50 percent.

A misfiring spark plug can reduce miles per gallon by about 2 miles; a sticky carburetor by about 3 miles; and a dirty air filter by about 3 miles.

* Keep an eye on the battery. The battery is what gets your car started and keeps it running. Proper care can save you from $45 to $60 or more. One quick way to see if the battery needs charging is to listen to the clicking of the turn-signal indicator.

A slow clicking that picks up when you accelerate the car means the battery may be low. Another way is to watch the headlights for low dim when the engine is off and bright when the engine is running.

Also watch for ''frosting'' which gathers on the top of the battery. Clean off any corrosion - a mixture of dirt, dust, and battery acid - with a stiff-bristled brush and a neutralizing solution of baking soda and water (about 2 tablespoons to a cup of water).

Keep the battery cells full with distilled water, easily bought at any auto store. Water from a defrosted freezer, a refrigerator, or a dehumidifier is also suitable.

* Shop for a mechanic. Pick a car mechanic or service station with utmost care. Look for skilled, reliable service at a fair cost. This type of dependable service could be a great saving to you in the long run.

* Buy your own parts. If you want to save even more money, take advantage of service bays that auto parts jobbers operate (see the Yellow Pages). These jobbers sell to professionals and individuals alike at prices that are 25 to 40 percent below the manufacturer's suggested list price.

A quick check of the Yellow Pages should show service stations and independent garages willing to install parts that are supplied by the customer.

Some do-it-yourself centers offer trained mechanic guidance or the use of tools at various hourly rates well below garage mechanic rates.

* Use auto care stations. Some gasoline service centers operate what is known as auto care stations. They verify that their auto care service is as professional as an auto dealer's but lower in cost. These auto care stations employ mechanics who are certified by the National Institute for Automotive Service Excellence to work on brakes, engine tuneup, front end, and heating as well as air-conditioning units.

Such an auto care station must meet requirements for appearance and management, guarantee service for 90 days or 4,000 miles, provide written estimates, and return replaced parts.

* Join an auto co-op. If you're fortunate enough to have an auto cooperative in your community, then sign up. For a modest lifetime membership fee, members get towing and road service at reduced rates as well as free shuttle service while their cars are being repaired.

They can also rent a do-it-yourself bay for a minimum fee on weekends.

The Co-op Bank Act, passed in 1978, entitles emerging co-ops to get capital advances and technical assistance. Auto co-op members operate their own repair shop for a nominal membership fee and elect a board that is responsible for operations.

Even though repair bills may be as high as at car dealer service garages, members feel they save because repairs are done right the first time, and they share in any profits and receive 5 to 10 percent discounts on parts, labor, or both.

* Read the facts. For only a few dollars, you can invest in a light service manual for your car. Clear illustrations and detailed explanations help any do-it-yourselfer diagnose car problems accurately. These manuals are available from auto dealers and parts suppliers.

* Check drive belts. Inspecting your car's drive belts can save you money and a lot of unexpected trouble. Those belts operate a car's engine-cooling fan, water pump, alternator, power steering pump, emissions-control pump, and air-conditioning compressor. If any one of the belts snaps, you could not only have an overheated engine or dead battery, but you could also be stranded.

In your inspection, first twist each belt and look and feel for signs of wear , such as cracks, fraying, or shiny spots. Press on the middle of each belt with your thumb. More than a one-half-inch depression means you need to tighten the belt.

Before buying any new belt, know the make and model of your car, engine size, serial and engine number, and the accessory the belt operates.

* Sign a service contract. A service contract offers mechanical insurance against major and expensive car problems.

* Bargain for the price. New-car prices vary from dealer to dealer. If you move fast after a price increase, you can buy from a dealer inventory before the cars with high stickers arrive.

To negotiate any price, you need to know the dealer's cost. You can determine this through Car/Puter, 1603 Bushwick Avenue, Brooklyn, N.Y. 11207; 800-221-4001 ; in New York call 455-2500.

State your preference for make and model. Car/Puter, for a cost of $18.50, will send you a printout with the dealer's cost for car and options. Also, Car/Puter will offer to obtain your choice at its recommended price, which can range from $40 to $50 over dealer cost for US-built cars in some areas (although a limited-production model might run up to $1,200 to $1,500) and 8 to 13 percent over cost for imports, through United Auto Brokers.

Make sure your local dealer will service the car afterward, however.

* Break in your car. You need to initiate your new car gently. While today's cars don't need as much breaking-in care as in the past, you can protect the engine significantly by holding down speeds, avoiding fast starts, and varying the speed at which you drive during the first few thousand miles.

* Read the owner's manual. Study the lists of recommended upkeep suggestions and avoid ''overmaintenance.''

Certain areas that require periodic lubrication on older cars may now be permanently lubricated and need no attention at all. Knowing this, and other facts found in the owner's manual, can save unnecessary expense.