Thinking of turning back the mileage indicator on your car before selling it?

Though plenty of Americans do just that in order to make the vehicle look like it's worth more, they had better stop it. Starting today, a new federal law is going to make it easier to detect offenders and get tougher with them.As matters now stand, the offense is rampant. Of the 20-40 million used cars sold in the United States each year, about three-million vehicles have had their odometers rolled back, according to the U.S. National Highway Traffic Safety Administration. Even new cars can have their odometers tampered with.

While odometer tampering already violates a federal law that has been on the books since 1972, the offense flourishes because it is easy to do, hard to detect, and highly profitable. Each 10,000-mile reduction in the figure displayed on the dashboard adds $200 to the seller's asking price. The average rollback is 30,000 miles.

The 1972 law has a glaring weakness. The law requires that a written statement disclosing the current odometer reading must be provided to the purchaser when a vehicle is sold. But the statute fails to specify how the seller must disclose the odometer reading. In many cases, Editorial Research Reports notes, the disclosure is made on a slip of paper other than the motor-vehicle title. When this happens, the buyer can easily destroy the disclosure slip, roll back the odometer, and prepare a fraudulent disclosure document for the next purchaser.

By contrast, the new law will require the mileage figure to be stated on all motor-vehicle titles at the time of transfer. It also elevates odometer tampering to a felony, raising the risk of prison sentences for violators who previously might have expected only a fine.

In addition, drivers of leased autos will have to disclose the true mileage to the leasing company when they return the cars, and automobile auctioneers will have to keep ownership and mileage records for four years after a car passes through their hands.

Despite this added protection, used-car buyers still should be careful. If the mileage on the odometer seems suspiciously low, look for such indications of odometer tampering as worn tires, upholstery, and brake-pedal covers plus scratches around dashboard screws. The new law, in short, is still no substitute for some old but good advice summed up in the phrase - "Let the buyer beware."