(MONEY Magazine) – When their new-car warranties run out--usually after three years or 36,000 miles--most motorists steer clear of the dealer's service department. Instead, they take the majority of their business to independent garages or national chains. That seems sensible when you consider that an independent mechanic will charge you an average of 15% less. And you'll usually find that savings at a convenient location close to home.

Lower bills are not the only reason car owners favor independent mechanics: In J.D. Power & Associates' annual driver satisfaction surveys, nondealer shops regularly top dealers for convenience, expertise and treatment of customers. One thing consumers say they like about independents is being able to talk to the mechanic who worked on the car, the shop owner or both, says Joe Ivers, who conducts the J.D. Power surveys. At dealers, on the other hand, consumers traditionally face a "service writer," who simply passes the order along to the mechanic.

Such unpopular ways of doing business are being replaced at some dealerships. Lexus and Infiniti, for example, let customers talk to the mechanic or shop foreman and train all dealers' employees in customer relations. Honda and Mercedes-Benz dealers, among others, have built satellite service centers close to malls and residential areas. These approaches can pay off. Lexus and Infiniti dealers outscore the average independent shop in J.D. Power driver satisfaction surveys; Infiniti says its dealers keep 65% of owners after their warranties expire--well above the industry average.

So where does this leave you if you need an oil change or find your transmission slipping? If your car is under warranty, you'll certainly want to take advantage of free dealer repairs. As for maintenance work that is not covered by the warranty, you're under no obligation to stick with the dealer. To make sure you do not violate your warranty, keep all your receipts for outside work.

In certain cases, you may want to go to your dealer--no matter the make of your car--even after the warranty has expired. Say you hit the 36,000-mile warranty cap in two years. An outside mechanic may not yet be experienced with your model, especially any computerized controls. Otherwise, "a well-equipped independent shop can repair your car just as well as the dealer service department," says Mark Eskeldson, author of What Auto Mechanics Don't Want You to Know and a mechanic who has worked for dealers and independents.

Wherever you go, keep in mind that overcharging, superfluous repairs and outright fraud have not been eliminated from auto repair. Sales contests sponsored by parts makers, unnecessary maintenance, incompetence and scams add 25% or more to Americans' repair bills, according to a 1995 National Association of Attorneys General report that looked at both dealers and independents. With that in mind, here's how to save at any garage.

Do background checks. As the table on page 183 shows, you have several ways to check out a shop. The National Institute for Automotive Service Excellence (ASE) certifies mechanics in eight areas (such as transmission or brakes) if they pass a test every five years and have at least two years on the job. As long as a shop has at least one ASE-certified pro, however, it can display the group's logo, so ask whether the person working on your car is certified for your repair.

Local affiliates of the American Automobile Association (AAA) compile lists of AAA-approved repair shops, including some dealer and chain shops but mostly independents. To make the list, a shop must, among other criteria, have an ASE-certified mechanic for all types of repairs offered and agree to binding arbitration in any dispute with an AAA member. Depending on local club policy, even nonmembers may be able to see the list.

Be leery of suggestions to replace parts. Some worn parts, such as radiator hoses and timing belts, should be replaced before they break, even if you haven't detected trouble. But in most cases you can wait until you hear or feel symptoms of a problem (or see a warning light on the dash).

Get a written estimate in advance. Hourly rates average $51 at dealerships vs. $43 at independent shops, according to the National Auto Dealers Association and the Automotive Service Association, a trade group for independents. Labor charges may be based on actual time spent working or flat rates from industry manuals, which often produce higher bills. Finally, you should also be suspicious of unusually low prices. In one scam, shops lure customers with coupons for cheap transmission tune-ups, tear apart the car and then drum up fictitious problems to repair that cost hundreds of dollars.