THERE ARE MANY differences between driving your own car and riding as a passenger in somebody else's, but it never occurred to us until last week that one of the differences might be the degree of protection you enjoy against illegal police conduct. If you own the car, the Supreme Court ruled last Tuesday, you have a right of privacy inside it that the police cannot invade without a good reason. But if you're merely a passenger, you have no right of privacy there at all. That, it seems to us, is a peculiar distinction. It gets even more peculiar when you consider where it could lead.

The case in which the court ruled involved a search of an automobile during which police found a shotgun under a seat and a box of rifle shells in a locked glove compartment. Two passengers, who were charged with robbery, tried to challenge the legality of the search but were told by the court they couldn't. They had no "expectation of privacy" inside that car, the court said, and without that they couldn't complain about the search. In trying to explain why, the court made it clear that the driver of the car could have challenged the search if she had been charged with a crime. She did have that necessary expectation of privacy because she owned the car.

Now that sounds as if the touchstone for privacy were ownership of the car. But the court's majority vigorously denied this when Justice Byron R. White made the charge in his dissent. The majority had no choice. After all, it had roundly repudiated the tie between privacy and property several years ago when it ruled that you are entitled to privacy in a public telephone booth. But the majority refused to elaborate now on what, besides ownership, gives you privacy inside a car -- or elsewhere.

That takes us to the complications. If the car had been a taxi and the passengers were paying a fare, an old court decision indicates they would have bought privacy. If the driver had given them keys to the car so they could use it, another old case involving the use of a friend's apartment suggests they would have been given privacy. If the passengers had been the children of the driver, there is a strong possibility they would have inherited privacy. If the driver had said, as the shells were locked into the glove compartment, "They will be safe here," she might have bestowed some of her privacy on her passengers.

Where all this will lead, nobody knows. We doubt that the justices will ever conclude that a visitor to your home is like a passenger in your car and has no expectation of privacy. While the analogy is close, the court regards homes more respectfully than it regards cars. And we have no idea of what it will do with a search of a mobile home or a van equipped with beds. Is it more like a car or a house?

The court got itself into this absurd posture because its majority hates to see suspects go free as a result of illegal searches. That happens sometimes because of a 60-year-old rule which bars the use in court of evidence seized illegally. despite pleas from some of its members to overturn that venerable -- and, in our view, vital -- precedent, the court has never mustered the votes to do it. So it has been chipping away at the number of cases in which evidence is barred by changing the rules to make more searches legal and, in this case, limiting the opportunities in which a search can be challenged.

The trouble is that the innocent as well as the guilty get searched. In reducing the number of guilty persons who go free because of police misconduct, the court is also reducing the pressure on police to behave properly.If they can win convictions after conducting illegal searches, some zealous officers will be tempted to forget about the legalities. Then the right of privacy that the court says an outomobile owner has in theory will simply disappear in reality.