Old or new, your car will run more economically if you drive right

Imagine an automobile that runs on pennies. Every time you step on the accelerator, a trickle of pennies slips from a reservoir on the dashboard into the carburetor where, somehow, it is changed into energy to run the car.

As you speed up, the trickle becomes a steady stream. If you touch the brake, you destroy the momentum it took so many pennies to create. Even while you sit idle at a traffic light, a slow clink, clink, clink continues. And when you bolt out of the intersection to beat out the guy next to you, a whole handful of pennies tumbles down the chute.

This somewhat exaggerated analog illustrates how driving habits affect the number of trips you make to the gas station.

While gasoline prices have leveled off and winter supplies are expected to be ample, saving pennies on fuel can still be important - especially if you've spent $8,000 or $10,000 on a new gas-thrifty automobile.

Thus, driving to prevent the gasoline pennies from tumbling down the chute is largely a matter of common sense.

The Motor Vehicle Manufacturers Association puts out a booklet that outlines simple ways to improve fuel economy. Among the tips:

* Avoid jack-rabbit starts. Imagine an egg between your foot and the accelerator pedal - and press gently.

* Don't idle unnecessarily. You get zero miles per gallon when standing still. On cold winter mornings, don't idle for a long time to warm up the engine. Rather, drive moderately until the engine is hot.

* Combine short errands into one larger trip. Your car gets the best mileage when it is at normal temperature; in other words, when the engine and tires are warm.

* Accessories, such as the heater and headlights, use fuel, too. Use only what you need.

* Higher gears get better mileage. A light foot on the accelerator pedal allows an automatic transmission to shift sooner into top gears. With a manual transmission, shift into higher gears at as low a speed as possible.

* Don't tailgate. You cannot drive smoothly if you have to constantly brake and accelerate. Keep an eye peeled for red lights ahead and give yourself time to slow down without braking.

* Obey the speed limit. Fifty-five miles an hour is not only safer, but it saves fuel as well. You get about 20 percent better mileage at 50 miles an hour than at 70.

Many cars today are most gas-efficient at about 40 m.p.h., according to Dick Hoover of the Massachusetts Division of the American Automobile Association (AAA).

Mr. Hoover also suggests removing unnecessary items from the trunk of your car. Why pay the cost of fuel just to carry golf clubs or bowling balls all over town?

As cars get older they begin to use more fuel and oil, he cautions; nevertheless, ''you can buy a lot of gasoline'' with the $5,000 you might spend to trade in your old car for a new one.

If you do decide to hang on to Old Faithful, be good to it - and keep the engine in tune.