300,000 miles from a car; armchair showroom tour; Why Trade It In? by George and Suzanne Fremon. Cockeysville, MD.: Liberty Publishing Company. $5.95.

First, this book doesn't tell the reader how to fix or adjust his car. In other words, it isn't just another do-it-yourself manual for the overall-clad motorist.

As the authors say: "It is intended to help these people keep their cars running indefinitely without having to do the work themselves."

hus, you will need a good, conscientious automobile mechanic to do the repair work on your car and keep it in tiptop shape. Working in tandem, the mechanic and the car owner can squeeze far more miles out of a car than even the manufacturer might expect.

That's the thesis of the book. Assuming the car did not leave the factory a lemon, the tips and guidance make a lot of sense.

This may not be the best time in history for a motorist to stay out of the market so far as the auto industry is concerned. US carmakers are in tough shape, more than 230,000 auto workers are on long-term layoff, and the prospects for the coming newmodel year aren't exactly buoyant. However, with the impact of infaltion striking everyone these days, if a motorist can get 8 or 10 years out of his car instead of 3, 4, or 5, it's money in the bank for him.

An automobile can go 300,000 miles or more, the authors assert, even if the car manufacturers talk about 100,000 as a mean.

The key to a long automotive life is preventive maintenance, chime the authors, and consistent, even driving habits by the motorist. Simply, you don't have to be a mechanic or even know how to use a wrench or a pair of pliers to get the most from your car and save thousands of dollars on the way.

The price of the book should be recovered fast if the reader pays attention to what he reads.

Proper tire care, for example, will extend the life of the tires by many thousands of miles. Yet how many motorists ever bother to check the air pressure on a regular basis?

While the book may not be for the engineer or highly sophiscated motorist who knows what makes the wheels turn and the engine run, most motorists do not fall into that category. One chapter lists some 75 symptoms which may give the driver pause, or stop the car altogether. The severity of the problem and what to do about it is spelled out in lucid detail.

"Why Trade It In?" or a book like it, the authors say, should be in the glove compartment of every car when it leaves the showroom. But this is the car owner's manual Detroit will not provide.