Shopping for auto insurance is still a pain in the butt.

I’d hoped it would be better by now. I’d hoped that the Internet would make the whole process more transparent. But you still have to check several Web sites and pick up the phone to call a few agents to get a truly comprehensive picture of what various insurers are charging. Some of the big companies don’t participate with online comparison services (which is why you have to visit their sites and, often, talk to an agent to get a quote).

Why would you go through the hassle? Because the differences in premiums can be huge–not just hundreds of dollars a year, but thousands.

That’s because insurers are all different. They have different policies and ideas about what poses a risk and how much of that risk they want to take. If they don’t want teen drivers, for example, they will make it extremely painfully expensive to add one. Other insurers will just make it painfully expensive.

Insurers also adjust their pricing to add or shed customers. If they want to get bigger in a certain market, they’ll chop their prices to attract more drivers. If they decide they’ve gone overboard, they will jack their premiums above their competitors to slow new applications. If you’re a long-time customer who doesn’t know any better, you could find yourself paying a lot more for the same basic coverage than you’d pay with one of those competitors.

If you want some incentive to start getting quotes, check out CarInsurance.com’s Rate Comparison Chart and then read Des Toups’ accompanying post, “The most and least expensive cities for car insurance.” The average premiums cited conceal a lot of variation, Des noted.

For example, the average rate from six major insurance carriers for ZIP code 48101 in Dearborn, Mich., was $2,522 — but that included rates as low as $1,776 and as high as $4,374.

Des ran the numbers for a ZIP Code closer to me–90025, or West Los Angeles. There the average was $1,915, but the range was from $1,106 to $3,136.

Price isn’t the only thing to consider, of course. How fast and how well the company handles claims matters a lot, too. Your state insurance commissioner may have complaint data that will help you figure out which companies to avoid, like this one at California’s Department of Insurance. The number to pay attention to is the “justified complaint ratio” which divides legitimate complaints by the number of policies the insurer has in the state. Just as there are big difference in price, there are also big differences in complaints.

In any case, you shouldn’t assume you’re getting the best deal. Every year or two, check around to make sure.