Presidio Sports Basement is the closest free parking but is often very full. I would try the Presidio Main Post area for metered parking, it's a nice short walk to the tops. Further parking is available at Chrissy Field and Marina Greens. All longer walks but on a nice day I don't think anyone would mind.
There is some limited free parking next to Sports Basement. Just put “presidio sports basement” in google maps and it will get you to it (there is an area just for clients but there are signs) Alternatively there is a large paid parking uphill. Just put “presidio tunnel tops” in google maps and you’ll get there.
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