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 (V) , MasterCard  (MA) 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.

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