A few days ago, I passed along a letter from a woman who had traded in a late model large car to buy a new small car that gets better mileage.

My comment was that not all such trades accomplish the immediate patriotic result that people hop for. In fact, the immediate consequence can, depending on circumstances, be increase rather than a decrease on our national expenditure of energy. Not gasoline alone, but out total energy needs.

It takes a lot of energy to manufacture a new vehicle and the components that go into it. When an old gas guzzler goes to the junk heap and is replaced by a compact, we save. But if the gas guzzler is a late model that some new owner will keep in service, it is not always clear whether there will be a saving or a loss in our current energy balance.

A few speed readers completely missed the point that I was talking about the immediate rather than the long-range effect. Some missed the point that it takes a lot of energy to manufacture a new car. One wrote, "Even somebody as stupid as you are should realize that every person who trades in a gas guzzler is patriotically helping to ease the crisis." Another began his letter, "In today's column, you were even dumber than usual."

Mike Silvia was one of several polite critics. "Sorry," he wrote, "but I think you're wrong." As he sees it, when my correspondent traded in her gas guzzler for a compact and somebody else bought the gas guzzler, we ended up with one compact and one gas guzzler on the road. If she hadn't traded it in, we'd have had two gas guzzlers. So we're bound to be better off after the trade-in.

However, other readers saw complications and ramifications that had completely escaped me. They look more carefully at the question of who buys that traded-in big car, and for what purpose. What was the buyer driving before he bought the used gas guzzler?

If he was driving a big car that was ready for the junk heap, the net change in the nation's gasoline consumption will be minimal. On the other hand, Charles A. Luckett pointed out that if he bought that used gas guzzler because it was cheap enough to let his family afford a second car, or third car, we may now need more gasoline rather than less. We have put another car on the road.

Robinson Newcomb raised the sae issue, plus another. He noted that if lots of people trade in big cars, used gas guzzlers will become a glut on the market. Their price will drop. People will be less likely to buy new luxury cars because the depreciation on them will become exorbitant. In the long run, that would save shiploads of gas.

An Annandale man who asked me to withhold his name also made a telling point. He wrote:

"I drive a big car that gets only miles to the gallon. I keep it well tuned, and am sure that it will serve me for many more years. It would be a silly waste of raw materials to junk it, so I am keeping it in service.

"However, to make up for the fact that this car uses a lot of gas, I now drive it sparingly. I have cut my pleasure driving to the bone, and have even taken to riding the bus whenever possible, something I never did before. Despite all this, a neighbor of mine considers me unpatriotic. He traded in his big car for a smaller one. Now he feels free to do an unlimited amount of pleasure driving, and recently returned from a long vacation trip to the West Coast. I would be willing to bet that he will use at least twice as many gallons of gasoline as I will this year because he feels no sense of crisis was war-related. What did he accomplish when he traded his big car for a small one? He's now burning up more gas than he did when he had the big one."

I think the point is well made. Buying a fuel-efficient car is only step one, and we have not yet gotten around to anything more than that.