Several District Liners have written recently to tell me, "To save energy, we have traded in our big car for a small one."

Some years from now, this trend toward smaller cars will no doubt produce a substantial saving in gasoline. But what about right now? Is there an immediate saving?

Here's a letter from a woman who writes: "We have traded in our beautiful 1975 Olds for a small car that gives excellent gas mileage, but I am heartbroken. We felt it was our duty to help the country save gasoline, but I sure do miss the joy of driving the big, wonderful Oldsmobile."

Her letter set me to thinking.

What happened to her "75 Olds after she traded it in? Did it stop using all that gas? Of course not.

Somebody else bought it, and is driving it, and is using just as much gas as the previous owner did. Meantime, a lot of energy was used up in producing the new small car my correspondent bought.

So the net saving for right now may be nonexistent - or even a minus quantity.

Switching to smaller, lighter cars that use fuel more efficiently will unquestionably lead to gasoline savings in the long run. But during this transition period, I think we have to retain our perspective.

If you are considering the purchase of a new car because your present big car is about ready for the junk yard anyhow, it makes sense to shop for an auto that will give you better gas mileage. But if your present big car has a lot of miles left in it, and the dealer who takes it in trade is going to pass it along to a new owner, I doubt that you do the nation any great service by "trading down" for the purpose of saving energy.

All you do is shift the burden of operating a 10-miles-to-the-gallon behemoth from one owner to another.