Auto insurers might need a supercomputer to process all the variables that affect your premium: driving record, claims history, credit history, age, gender, amount and type of auto insurance purchased, where you live, and the type of car you drive, just to name a few. And while the whole picture is greater than the sum of its parts, the vehicle you choose can have a substantial impact on the car insurance premium you pay, especially if you purchase comprehensive and collision coverage.

According to the National Association of Insurance Commissioners (NAIC), comprehensive and collision coverages accounted for 48.5 percent of the combined average premium paid nationwide in 1999 (the most recent data available). The average collision premium in 1999 was $249.36, and the average comprehensive premium was $130.82.

The combined average premium includes liability, comprehensive, and collision coverages. Comprehensive and collision insurance premiums are calculated by looking at vehicle loss history — that is, how often that model is stolen and how much it costs to repair or replace it after an accident.

Generally speaking, your car model does not affect the premium you pay for liability insurance alone, but there are exceptions. Sports cars, high-performance cars, and intermediate-performance cars as designated by the Insurance Services Office (ISO), a statistical and actuarial reporting group for the insurance industry, might cost you more in liability premiums if your insurance company has adopted the ISO's vehicle designations.

The ISO publishes a manual used by insurers and agents that rates each vehicle based on its loss history — or its manufacturer's suggested retail price if the model is brand new. The ISO assigns each vehicle a number between 3 and 27 to represent the cost of comprehensive and collision coverage. Auto insurers can use the ISO's vehicle-numbering system to calculate the car's comprehensive and collision coverage costs. The higher the number, the more expensive the coverage will be. Unfortunately, this manual is not publicly available.

SUVs take you higher

In addition to sports cars, sport utility vehicles (SUVs) might also cost you more in liability premiums. Some companies have decided to rate SUVs differently based on the vehicles' histories of causing more damage to other vehicles when involved in collisions. Steven Lehmann, a consulting actuary for Bloomington, Ill.-based actuarial firm Miller, Herbers, Lehmann, and Associates, says that because Allstate and Progressive are charging vehicles differently when it comes to liability insurance premiums, more insurers might follow suit.