Sen. Wendell Ford (D-Ky.) succeeded the other day at one of the tasks that regularly challenge legislators. He drew the attention of the major media to his subcommittee's hearing by announcing that its subject, odometer fraud, costs American consumers an estimated $1 billion a year.

1,000,000,000 a year. A nice, round figure, one that carries in its nine zeroes more "oomph" than a news organization can ignore.And most didn't.

It's the kind of figure seen often in newspapers, heard frequently on the evening news. It catches your attention, convinces you that here is a major problem, deserving your concern.

But where do the people who give these figures get them?

An aide did much of the senator's background work for the hearing on the widespread practice of resetting mileage counters to make cars seem less used than they are. She is not quite certain where the $1 billion figure came from.

"To me, it's all guesstimate," she said. "Purely a guess," she added later.

After a moment to think about it, she said she thought her boss was quoting a figure supplied by the Transportation Department's National Highway Traffic Safety Administration.

But, she cautioned, NHTSA has only one investigator, "and they really have no statistics.What I think is that they base it on statistics from the individual states."

At NHTSA, administrator Joan Claybrook said "no one knows exactly" how much odometer fraud costs the public. Her agency's estimate is not $1 billion, she said; it is "between several hundred million and one billion."

How can she be so certain? "It's possible to draw some fairly accurate conclusions" from prosecuted cases and negotiated settlements, she said.

Indeed her figures are based on state studies. Claybrook said, but she cautioned that where state laws are inadequate there has been no enforcement.

For better information, she said, contact her assistant chief counsel, John Womack.

Well, said Womack, here are several paths to the $1 billion estimate; one leads through the state of Washington.

Five years ago, Womack said, the Washington Department of Motor Vehicles was receiving a large number of complaints from consumers who had purchased used cars with 20,000-mile odometers but 60,000-mile engines.

Under cover of night, Womack said, state agents visited a number of used-car lots, peered with flashlights at licenses, serial numbers and odometers of cars waiting to be sold, then checked with former owners to learn if the mileague had been changed.

They found that about 20 percent of the odometers had been reset, or "spun," by an average of 30,000 to 40,000 miles.

Another path led through California to a somewhat lower estimate, Womack said. Using mileage records on the titles of cars bought in California but sold out of state, California officials determined that odometer fraud occurs in between 10 and 15 percent of used-cars sales.

In the Blue Book that estimates the value of used cars based on age, condition and mileage, Womack said, a difference of 10,000 miles on an average American-made car is worth about $100.

About 14 million used passenger cars were sold last year, Womack said. Armed with a computer and the figures from Washington and California, he said, one could project the cost of odometer fraud at anywhere between $560 million, using California's 10 percent figure, and $1.1 billion, using Washington's 20 percent.

However, here is the man who put together the Washington study, Department of Licensing official Robert Hayter:

"That billion dollars I consider to be conservative. Very conservative. All you can do is take a sampling, but I could open up my files here and show you . . ."

Now, none of this is to say that odometer fraud is not a serious matter. It probably is. But that nice round figure they attracted attention with could apparently as easily have been half a billion, or two billion. Then again, without the attraction, maybe no one would have gone to Sen. Ford's hearing.