If you just purchased a new car, you undoubtedly are aware its value dropped significantly as soon as you signed on the dotted line.

"The moment you drive your new car out of the dealer's lot, you are leaving some of your cash at the curb," said Joseph Ferrau, director of claims for Metropolitan Property & Liability Insurance Company in Warwick, Rhode Island. "It's unfortunate, but new cars depreciate the moment you buy them. After only one month, a $15,000 to $17,000 car may depreciate in value anywhere from $1,500 to $5,000."Your insurance policy probably will not cover the replacement cost of your car if it is destroyed or stolen soon after you take possession. Most likely, your insurer will only pay a claim equal to the car's present retail value.

To make matters worse, if you only made a minimal downpayment on the auto, the outstanding loan balance may actually be greater than the retail value of your car. This is known as "being upside down."

If you are caught in this value gap and your car is stolen or totalled, the insurance check will go to your lender, and you still would have to make good on the rest of the loan.

"This is a risk the public does not see when buying a new car," said Sam Barash of CCC Information Services in Chicago, which supplies data on used car values to the nation's major insurance companies and auto dealers. "You may think you're fully covered, but insurance will not cover you completely if you have financed the vehicle and the current loan balance is higher than the present value of the car."

A growing organization of U.S. auto dealers is planning to offer memberships in a new consumer car club, Priority Plus. Among the features to be included in membership is "gap protection." Gap coverage will prevent your having to take a cash loss if you lose your new car during the early period when your loan balance may be higher than the car's current worth.

Priority Plus also will offer discounts on repairs and road service, free towing and periodic maintenance reminders. The club is seen as an organization that helps consumers protect their cars and their investments. After a home, a new car is the largest investment most people will ever make.

Readers who want more information on gap protection should contact an authorized Priority Plus auto dealer or write Priority Plus, P.O. Box 7119, Chicago, Illinois 60680, or phone (312) 662-5300 and ask for Gary Sookikian.

STEER CLEAR OF DEPRECIATION POTHOLES

The value of a used car is based on a number of variables, many of which you can control. For example, a well-maintained auto will always be worth more in the long haul.

"Resale values depend on the overall condition of the car, with equal weight placed on the vehicle's appearance as well as what's under the hood," said Frank Dremon, a used car salesman with the Perillo Chicago car dealership.

Besides following a regular maintenance schedule, it's important to pay attention to aesthetics. Auto experts point out that nothing hurts the resale value of a car more than rust problems or an accident.

Those in the business of assessing auto values even rate used cars according to a scale of four categories: extra-clean, clean, average and rough.

For example, a 1986 Olds Cutlass Cierra SW originally cost an estimated $11,257. A year later, if the car was in "extra-clean" condition, the resale price would have been $8,050. If the car was in "clean" condition, the value would have dropped to $7,500. A "rough" 1986 Cierra would have fetched only $5,100 in 1987.

However, despite the importance of maintenance, age and mileage are the two key factors determining your car's present worth. "Even if the car looks good on the outside, it's the mileage that counts the most when you're selling," said Dremon.

The insurance industry standard for determining depreciation roughly says that every mile lowers a car's value by 10 cents.

Here's a list of other variables the used car industry considers when you're selling or trading-in your car:

- Optional equipment. Extra options can add to your car's resale value. But air conditioning and convertible tops may get a cool reception from residents of some of our northern states.

- Public perception. Though your car may still purr like a kitten, public opinion concerning the model's past performance or present pricetag may dramatically affect its value.

- Regional and geographic effects. Used cars in warmer climates retain more of their original value, because they are not exposed to the harsh weather and road salt experienced by that their northern cousins.