Actually, if you're only going to be driving on Key West you really don't want a car if you can avoid it. Parking in many areas can be scarce, and the use of bicycles, scooters and golf carts is common. However, if you plan to leave Key West (i.e. drive off Key West to other Keys) then you need a car.
I was elsewhere in Florida a few months ago and drove down the Keys to Key West for a one week vacation. Parked the car for a week and didn't move it once.
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