Both is possible with the Discover it® Miles card. When you use the card to rent a car, you will earn 1.5 miles for every dollar spent. To use miles, you should rent a car with your card and when the purchase appears on your online account apply miles to it. Or you can redeem your miles for cash, and use it to rent a car.
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All rates and fees, and other terms and conditions of the products mentioned in this article/post are actual as of the last update date but are subject to change. See the current products' Terms & Conditions on the issuing banks' websites.