The average cost of auto insurance will rise at a faster rate in 1980 than in 1979. Profits have slowed in the property and casualty business, which means, sure as night follows day, taht insurance premiums will rise. Industry estimates lie in the 5-to-7 percent range, compared with an average increase last year of 4.2 percent.

If you drive a small or sporty car, your insurance costs may rise above the average -- if not this year, then next year or the year after. Many auto insurers are leaning toward charging more for cars that are easily damaged, often stolen, or cost more than others to repair.

Alstate pioneered this approach in 1976, followed by Geico in 1977. The Insurance Services Office, a national insurance advisory and statistical organization, also has developed a model car-rating system, approved so far by 30 states. ISO is gathering data on the repair costs of various models, which it will feed to the many insurers considering the plan.

Geico's better-rated cars, with lower insurance costs, include the station-wagon and four-door models of the Buick Apollo, Skylark, Electra, La Sabre and Century; Chevrolet Caprice, Impala, Belair, Chevelle and Nova; Dodge Aspen, Dart and Monaco; Chrysler Newport and New Yorker; Oldsmobile Delta 88, 98 and Omega; Plymouth Fury, Gran Fury, Satellite, Valiant and Volare; and Pont Catalina and Le Mans. Also, the two-door Skylark, Impala and Oldsmobile 88.

Their lowest-door models, are the Chevrolet Camaro, Corvette and Monza; Pontiac Firebird and Gran AM; Volkswagen Karmann Ghia, Rabbit and Scirocco; Capri Sport; Oldsmobile Starfire; Buick Skyhawk; AMC Javelin; Dodge Challenger; Plymouth Barracuda; Ford Mustang II and Thunderbird; Lincoln Continental Mark IV; Mercury Cougar; and Datsun's "2" models. Also, the Porsche, Alfa Romeo, Jaguar, MGB-GT, Triumph TR6 and TR7 and Lotus.

Not all smaller cars are poorly rated. Elton Stephenson of Allstate told my associate, Dedra Hauser, that the Chevy Citation, Pontiac Phoenix, Buick Skylard and Oldsmobile Omega are all cheaper than average to repair and hence have slightly lower insurance premiums. Allstate's better-rated list even includes the sporty Chrysler Cordoba and Pontiac Grand Safari.

Aetna Life & Casualty is just starting to use the ISO vehicle rating program. Its better-rated cars, 1980 models only, are the Buick Electra, Chevolet Impala and Malibu, Dodge Aspen, Ford LTD, Pontiac Le Mans, Oldsmobile 98 and Toronado and Plymouth Volari.

Two cars low on the ISO list are the Chevy Corvette and Pontiac Grand Prix. According to the National Automobile Theft Bureau, they're the most stolen cars in America. The Corvette and VW Scirocco are the most expensive to repair.

Vehilce rating programs are still the exception, not the rule. Although many insurers think them the wave of the future, others aren't so sure. State Farm Mutual Insurance, for example, says that it's not seriously considering such an approach.

Another change in insurance pricing relates to your car's age. Some companies no longer automatically lower physical-damage premiums as the car grows older and less valuable. The theory here is that the price of a new bumper is approximarely the, regardless of the age of the car. (If the car is totalled or stolen, however, you can collect no more than the depreciated book value, even if insured for more.)

To the auto buyer, this flurry of pricing changes means one thing: The rate discrepancies between one insurance company and another have become larger than ever.

You can save yourself some money by shopping around, not only for companies with vehicle rating plans that favor your paticular car, but also for insurers with a low, general rate structure. The pricing differences among companies are so great today that it's possible to pay twice as much for auto insurance than you really have to.

The easiest source of information on rate differences among companies is general insurance agents, who do business with more than one insurer.