If you have liability insurance you are covered, but only for liability. Liability insurance basically covers the driver and passengers but not the rental vehicle itself. Liability insurance covers you in whatever vehicle you drive, as long as driving that particular vehicle is a non-common occurrence or not part of your normal driving routine.
Your insurance company will cover any damages you cause to anyone else (property and injury), up to the minimum legal limits required by law or the limits on your policy,if different.
Many drivers are covered in some way for an average car rental. Talk to your insurance agent. The same coverage and deductibles you carry on your car will apply - in most cases - to a vehicle you rent. Be careful. If your policy provides minimum coverage, you may need to calculate the value of the loss or damage of a brand-new vehicle (the rental car) and decide if you need to supplement it.
If you decline LDW at the rental counter and rely on your personal coverages, you probably would be responsible for paying your usual deductible. Also, a loss-of-use fee is not normally covered by personal auto insurance policies. And, personal auto policies that extend collision coverage to a rental car may only provide a coverage limit equal to the value of your own vehicle. If the value of your personal vehicle is less than that of the rental vehicle, you could still incur some damage responsibility.
Loss of use is a fee that is charged by the rental company if your rental car is damaged and is sent out for repair. You would be billed for the complete cost of the car rental's repair as well as loss-of-use fee for every day that the vehicle was not able to be rented. That can add up!
You should also check to see if you are covered for damage to the rental vehicle through you credit card.
The bottom line is check into everything. You can certainly save money by declining the optional waivers sold by the car rental companies, but only if you are already covered!
Source:
Buying Rental Car Insurance