Even if your personal car insurance covers you both while driving in your own vehicle, it will not cover both of you in a rental car, unless both of you are legally entitled to drive
the vehicle -- both drivers must authorized by the car rental provider that you
decide to rent from.
It is important to follow the rules, regulations and terms and conditions of your rental in order to validate any coverage you may personally have, either with your own insurance company, credit card coverage, or coverage purchased from the rental provider.
We have a lot of information on our site about various policies, procedures and guidelines for major rental car companies.
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Added Driver Charges
Unfortunately, most of the time, the authorization of an additional driver, also comes hand-in-hand with a charge for a second driver. (Some car rental companies do not charge an additional driver fee for a spouse, life partner, common-law spouse or a fellow employee (when on a business rental). Some companies charge for any extra driver, other than the renter.
There are two schools of thought on WHY the car rental companies charge for a second driver.
1) Because they can. Many people feel that there is no actual reason for the extra charge; it's just a way for the car rental company to make more money. See this article, for example: 5 New Car Rental Fees to Avoid, by Christopher Elliott
2) The extra additional driver fee is a way for the car rental companies to recoup the costs associated with having multiple drivers on their cars. Most rental companies, in most places, provide basic public liability and property damage coverage for its vehicles and their renters and authorized drivers if required by applicable law. (California and Texas are common exceptions; check with your local rental car provider). Sometimes this only applies if you do not carry your own personal automobile insurance coverage.
Read more about: Car Rental Liability Insurance
I searched for the answer to your charge for a second driver question, with several car rental companies. I did get a response from Rob White, who is the Marketing Manager for Avis UK, and a moderator for the Avis We Try Harder blog.
Here is my question and his response:
*Note: Car rental companies can modify policy at any time; I make every attempt to maintain up-to-date info, but always check with an individual company to verify any policy. See: Disclaimer