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My question is are the outside diameters of the 20's about the same as the 18" spare? I think the 20" rims just have lower profile tires and the outside measurements are all within a half inch. Otherwise Rich is right you wouldn't want to go far with the spare on the truck. I suspect they are pretty close and Ford only does it to save a few$$$.
As for the original question I have at least 6 inches as a minimum as I am one of those people who have rubbed the tops of the bed while leaving a particular campground we frequent.
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