An engine tuneup performed at regular intervals is essential to top mileage and performmance. Ignition timeing off just 5 degrees from the proper setting can cost you half a mile per gallon as well as contribute to decreased engine performance. One spark plug misfiring at 55 m.p.h. can cost you over one mile per gallon.

How often a tuneup? On cars with breaker-point ignition systems, 10,000 miles. On new cars with electronics ignition, less often.

Unfortunately, the price of a tuneup, like everything else, is going up - $45 or much more.

Can you do your own? Yes. But not with just a couple of screwdrivers and an adjustable wrench. If you don't already have the equipment, you'll have to figure on laying out about $100 or more. You'll need:

TIMING LIGHT. Used to check and adjust ignition timing. Easy to use and comes with instructions on how to hook up and operate. Get the gun-type light with two wires - one hooks to the battery and the other to a spark plug wire. $15-$30.

DWELL-TACHOMETER. A satisfactory one runs under $30. Used to check the time the ignition points stay closed (what the pros call "dwell") and to check and adjust engine rpm. For example, when you check and adjust ignition timing, the engine must be running at a certain rpm. The tachometer gives you the rpm reading. Dwell-tachs come with instructions and are easy to use. Just hook up two wires and take a reading.

VOLT AMMETER. Under $40; measures voltage and amperage. Comes with instructions on how to take various readings as specified in your shop manual.

OHM METER. Checks out resistance of spark plug wires to determine if they are bad. Often you'll find the ohm meter combined with another meter, such as the volt meter. In fact, you can buy what's called an automotive analyzer or multi-tester that contains voltmeter, ammeter, tachometer, dwell meter, ohm meter in one unit. This is one way to save money, because it is usually cheaper than all the instruments purchased individually. Under $100. The two biggest disadvantages to them are that they can be bulkier than a single instrument, and because so many scales are printed on their face, they can be hard to read.

COMPRESSION GAUGE. Determines cylinder compression. It too comes with instructions. You can't do a good tuneup unless the compression in all cylinders is adequate. And the only way to tell is with a compression gauge. The best kind is one that screws into the spark plug hole, not one that you must hold with your hand against the hole. A good compression gauge runs around $30.

SPARK PLUG GAUGE. Around $2 or $3. You need it to check and adjust spark plug gap to the specs called for in the shop manual.

HYDROMETER. Checks battery's state of charge. You can get the cheap ball type for a buck or two. More precise ones with a scale and built-in thermometer run $5 or more.

BATTERY POST CLEANER. Used to clean battery posts and cables. About $2.

In addition you'll need a few hand tools screwdrivers and wrench or two.

How do you do a tuneup? After you've assembled all your equipment, read their instructions and become familiar with their hookup and operation. Then read the section in the shop manual that deals with tuneup. Then if there's anything you don't understand, before you begin the tuneup, scoot down to your local garage and ask a mechanic to explain it to you. Then you're ready to begin.