NEW YORK (MainStreet) — Credit-card companies like to tout how you'll get free insurance if you rent a car using their card, but a detailed analysis finds that the coverage they provide sometimes excludes big SUVs, overseas rentals and more.

"Most people probably have at least one card in their wallet that provides car-insurance coverage, but what's important to understand is that the devil is in the details," says Odysseas Papadimitriou of market tracker CardHub, which looked recently at what Visa, MasterCard and other card firms actually cover.

Car-rental firms offer customers extra insurance to pay for damage that a vehicle sustains, but savvy consumers know that credit-card companies often throw in free coverage if you pay for the transaction with your card. (The insurance policy you have on your own car often extends to rental vehicles as well.)

While that means renters can often skip added insurance — which typically costs an extra $10 to $30 a day — CardHub found that there's plenty of fine print when it comes to what exactly card companies cover.

To see who pays for what, CardHub graded American Express, Discover, MasterCard and Visa's offerings on everything from whether they insure against tire damage to how many countries (if any) they exclude from their coverage areas.

Some findings:

  • American Express ranked No. 1 overall for coverage, followed (in order) by Discover, MasterCard and Visa.
  • All American Express, Discover and Visa cards include car-rental coverage, but not all MasterCard accounts do.
  • American Express is the only card issuer that excludes certain popular sport utility vehicles from coverage. These include the Chevrolet Suburban, Chevrolet Tahoe, full-sized Ford Bronco, Ford Expedition, GMC Yukon, Lexus LX450, Lincoln Navigator, Range Rover and Toyota Land Cruiser.
  • Visa is the only card company that doesn't cover accidents that occur on dirt or gravel roads, while MasterCard will pay for such incidents only if the road in question is "regularly maintained." 
  • None of the card issuers cover rentals of exotic/antique cars, off-road vehicles or trucks (including pickup trucks).
  • All four companies exclude "long-term" car rentals, which typically mean those that run for more than 15 or 30 days. (The limit depends on which card you use.)
  • Card issuers often don't cover rentals that occur in certain foreign countries, including some that large numbers of U.S. consumers visit. For instance, American Express doesn't cover vehicles rented in Australia, Ireland, Israel, Italy, Jamaica and New Zealand.
  • All four issuers will cover your rental only if you pay for the entire transaction using their card. You also have to decline the optional insurance the rental company offers you.

Papadimitriou says the best way to make sure you're covered for a given rental is to call your auto-insurance company and credit-card issuer ahead of time and tell them exactly what you plan to do. Include details of where you're going, how long you're renting for and what vehicle you plan to use (you can reserve a specific model in advance).

"You don't want to be calling when you're at the rental-car company's counter with a huge line behind you," he says. "That's how you end up buying coverage that you don't really need."