Keeping secrets is fine when you're in grade school, but when it comes to taking care of your car, secrets are just what you don't need, especially from your insurance company. Here are 12 things your auto insurer won't tell you, and what you can do about it.

1. How they determine your car's value after it's declared a "total loss."
Insurers say they use three mechanisms to determine the value of a totaled vehicle: computerized vendor quotes, value books such as Kelley Blue Book, and a market search of the local area. However, the local area isn't specifically defined and an insurer may be unable to locate a replacement car within your neighborhood. If an insurer finds a replacement car outside of your living area, the valuation can be affected.

2. You may be entitled to payment for sales tax and registration fees for a new car..
What you can do: Count on having to make the request; don't depend on the insurer to offer to pay up front.

Even in states that do not require sales tax reimbursement, you should request it. Many auto insurers will not deny the request because the policy requires that they make you "whole," which means you should recover all of the costs for returning you to where you were before the accident.

3. You may be entitled to a diminished value claim in some states.
Diminished value is based on the idea that any car that has been in an accident, regardless of how well the repairs are done, is worth less than the exact same car that hasn't been in an accident.
 
4. You may be able to "stack" your coverage.
Stacking uninsured/underinsured motorist coverage's means you can collect from more than one auto insurance policy that you hold. Most states forbid this practice, but 19 states either don't address the issue or allow stacking. See Stacking your UM/UIM auto coverage.

5. How much making a claim could increase your rates.
Many insurance companies follow an industry standard of increasing your premium by 40 percent of their base rate after your first at-fault accident. So, for example, if the company's base rate is $400, your premium will go up by $160. Not all auto insurers play by this rule, though, and some may increase your individual rate by 40 percent. Regardless of what formula they use, in the majority of cases, your rates will go up.

6. Your credit history can dramatically affect your auto insurance premium.
According to a 2001 survey of the top 100 insurers in the country by Conning & Co., 92 percent of responding insurers use credit information to create an "insurance risk score," which they then use as a factor to determine your auto insurance rate. The theory is that there is a direct correlation between your insurance risk score and the likelihood that you will file a claim. Insurance scores are intended to evaluate your stability, meaning if you pay your bills in a timely fashion and have had the same credit accounts for long periods of time, you're considered more stable than someone who pays late or sporadically and who opens and closes accounts frequently.

7. You must officially cancel your insurance policy when you switch insurers.
Most auto insurance companies state in your policy that you can cancel your coverage at any time by notifying the company in writing of the date of termination. However, most consumers assume that if they decide to terminate the policy at the end of the coverage period, all they have to do is ignore the bill. The insurance companies don't see it that way. They will send you another bill for the next premium payment, and when you don't pay it, the company will cancel you for nonpayment, which goes on your credit record.

8. You can wait to add your teenager to your policy until he or she is licensed.
In most cases, insurance companies don't require you to add your teenager to your policy until they have their driver's license. The exception may be if you are in a high-risk pool; you may then have to add your child when they receive their permit.

9. Paying in installments will usually increase your overall bill.
"Fractional premium" fees are usually charged when you divide your annual premium payment into installments rather than pay for a year of coverage all at once. Payments are usually offered on a six-month, quarterly, or monthly basis, but almost every insurance company charges an administrative fee for breaking up the payments. It can be as little as $10 per payment, but the more you break it down, the more it adds up.

10. How much your car model affects your premium.
You won't get these numbers from your insurer; in fact, you may not be able to get them at all. But the auto insurers do have a premium rating system for every car model, based on ratings received from the Insurance Services Office. Cars are rated from three to 27, and the higher the number, the higher your premium. The ISO says it won't release the numbers to the public because its employer is the insurance company, not the consumer.

11. You'll pay for your friend's bad driving.
If your friend borrows your car and crashes it, you'll have to file a claim with your insurance company. You'll have to pay any deductible that applies, and your rates will probably go up as a result of your claim.

12. Your personal property in your car isn't covered by your auto insurance.
Stolen or damaged items like compact discs aren't covered by your auto insurance.