The owner of that 1972 Chevy with the dented left rear fender parked among the elegant limousines lined up in front of the Pentagon's prestige entrance is the spokesman for the deadliest federal engine of all, the Department of Defense.

His official title, assistant secretary of defense for public affairs, gives Thomas B. Ross four-star status when he lands on a military base. Pretty heady stuff for a onetime wire service reporter.

Yet the five-sided building where Ross spent the last four years as spokesman is a paradoxical place, where status does not always mean power.

"You know," Ross said in a valedictory of sorts the other day, "I'm in charge of 300-plus people inside the building and 3,000 public information people worldwide and a budget of $50 million. Yet I can fire exactly two people, my civilian deputy and my secretary."

Such is the nature of the Pentagon. By day there, generals and admirals make recommendations to the president which would mean incineration for the whole world. By night, a spirited group of ordinary people takes square dancing lessons in the cavernos shopping complex in the basement. One store in that complex sells sweet buns and another pantyhose not far from "the tank" where the Joint Chiefs of Staff get top-secret briefings on mischief around the world.

In short, the Pentagon is a crazy place, depressing for many who work there, like those colonels who used to run whole bases in the hinterlands and now are treated like errand boys among the generals. Former defense secretary Donald Rumsfeld once pedaled a messenger bicycle around the Pentagon to see what its outer reaches looked like. "It's so depressing," he said after cycling up and down the sterile-looking corridors.

But Ross said he came to love the place in four years of wrestling with its contradictions. It doesn't hurt when he laughs. He seemed to laugh more than any of his predecessors in the job of spokesman.

There was Arthur Sylvester, who became the embattled defender in the Democratic 1960s of the doctorine that "the government has the right to lie."

There was Phil G. Goulding, who sat by poker-faced as Secretary Robert S. McNamara called reporters into his dining room and tried to portray the USS Liberty as something other than a spy ship.

Jerry W. Friedheim stood up day after day grimly trying to defend the Nixon administration's continued involvement in the Vietnam war it had promised to end quickly; the bombing of North Vietnam's Bach Mai hospital was one of his memorably bad moments.

And Jospeh Laitin had the controversial rescue of the merchant ship Mayaguez, captured by Cambodia, to defend.

Compared with them, Ross had it easy. He did unwittingly lead reporters to the conclusion that Defense Secretary Harold Brown would recommend putting some B1 bombers into production, and found himself among those frozen out of information on the Iranian rescue mission.

But, partly because the United States stayed out of the war during his four years, Ross leaves the job of assistant secretary of defense with no serious wounds. He is even liked by many of the reporters he deals with day by day. And he says he feels he drove this federal engine in the right direction by adroitly using two powerhouses: Secretary Brown and Gen. David C. Jones, chairman of the Joint Chiefs of Staff.

Something that still works in this government is the military chain of command. Shortly after taking office Ross concentrated on demonstrating to generals, admirals and bureaucrats that he had the full backing of Brown and Jones in asking for information to pass on to reporters, setting up interviews and suggesting how to handle problems.

With this help from the top, Ross says he got rid of much of "the bunker mentality" that he found in civilian exeuctives and military leaders when he reported for duty at the Pentagon in 1977. "They were still under the feeling that the public, and particularly the press, held them to blame unfairly for the defeat in Vietnam," he said.

Partly because of the progress he made in erasing mutual suspicions between the military and the civilian community, particularly the press, Ross said he fears that the pendulum is swinging too far the other way.

"The military is getting such a favorable hearing that there is a danger that everything that drops from their lips will be taken as holy writ," he said. He said he feels this was the case during the presidential campaign, when, partly through widespread leaking of secret documents on the state of U.S. forces, the ordinary reader "got the idea we couldn't fight our way out of a paper bag."

Ross said he is proudest of his success in opening up the Pentagon to the press, and most disappointed about failing to get across to the public that it was the Carter administration which got the nation on the present upswing in defense spending, even though Ronald Reagan managed to make it look otherwise.

Ross' critics among military information officers complain that he spent almost all his time worrying about Harold Brown and outside consumers, newspaper and television reporters, rather than helping to improve communications within the military services. "Tell him to come down here and learn our names," one such military critic said.

Ross acknowledges that there is substance to such criticism, but said the top information officer cannot do everything himself; that he tended to delegate dealing with intra-service communications to his military deputy and concentrate on being Harold Brown's spokesman and on policy questions. Given his authority to fire only two people, Ross said, "You don't have to be a genius to figure out you're not going to make much of a dent" in the military information apparatus, even if one concentrated on it.

Summing up, Ross, 51, former Washington bureau chief for The Chicago Sun-Times, said, "I'm proudest that I've had an honest record. I've gone through four years without consciously misleading people." As for his recommendation for improving that big part of the federal engine he has come to know from the inside, Ross said:

"My sense after four years is that of someone who has begun to long for the spoils system."