Only the Diner's Club card is primary when it comes to car insurance and yes, you must decline their insurance (at least not agree to anything extra). It is the only thing I use my DC for, and since the fee is waived because of my citigold account it is a no brainer to keep it. I was in a car accident in England about 5 years ago - a head on collision that I caused. DC totaled the car and paid $14,000. It cost me nothing - and didn't go on my driving record. Needless to say I would never drive in England again and I would only use DC for my car rentals. Always good to call the credit card that you are using to clarify what is covered.