According to customer FAQ on Amex.com
I went to https://faq.amextravel.com/app/answers/list/c/2219,2306/kw/Membership%20Rewards%20Car
and clicked on the link for the question "can I use membership rewards for car rentals".. it states:
At this time, it is not possible to use Membership Rewards® points for car rental reservations. This is due to the fact that payment for most car rental bookings occurs upon return of the car and bookings are not pre-paid.
So this isn't saying you can't use the points for car rentals, just that you can't use them to reserve the car.
Then I clicked on the question "In the past, I have redeemed membership rewards points for car rentals... am I still able to do this today?
You still have the ability to redeem points with our travel partners by redeeming points for car rental certificates. For more information, please click here. (which takes you to a page where you have to log in to your amex account to see what is available for you to "purchase" with your points.
What you should do is see what certificates are available to you to redeem and then carefully read the terms and conditions of use and follow any instructions that are required to use the certificates. The car rental company that honours the certificates may have special requirements for their use.
The first step should be to contact customer service for amex to clarify what you can and can't do.
The phone number for Flight, Hotel, Car Online Support is 1-800-297-2977
You would have to double check the terms and conditions provided for either the rewards certificate directly, or the terms and conditions set out by the particular car rental company that is allowing you to redeem the rewards.
*note: you can redeem American Express Membership Rewards Points for coupons/certificates for Avis, National or Enterprise.
... they wanted to know if they could use a different Avis Discount Code (AWD number) with the coupon number on the Amex certificate. I went to the Avis website and did a search for Amex MR (Amex membership rewards). (you would now have to go directly to Avis.com/amexMR ) I was able to find terms and conditions for this program that confirmed that one of the terms and conditions listed stated (under the terms and conditions link at the bottom of the the box to the right)
"Offer may not be used in conjunction with any other AWD#, promotion or offer."
You may find it easiest to contact a customer service representative, by phone, for the particular car rental company that you are going to rent with, and ask them if they can provide you with information on the terms and conditions specific to using American Express Car Rental Rewards Certificates with them in particular. The certificate itself may already contain rental car discount codes that are to be used with the Amex membership rewards.