Unroll the piecrust into a 10-inch fluted tart pan with a removable bottom. Press the crust against the bottom and sides of the pan, trimming the top as needed so it is even with the rim. Refrigerate.
Position a rack in the center of the oven and preheat to 425 degrees . Peel, core and cut the pears into 1/2-inch-thick slices and place in a large bowl. In a small bowl, mix 1/2 cup brown sugar, 2 tbsp. flour, the apple pie spice and salt. Toss the brown sugar mixture with the pears to coat, then spoon into the prepared piecrust.
In a medium bowl, stir together the remaining 1/4 cup brown sugar and 1/4 cup flour. Using a pastry blender or your fingers, cut in the butter until the mixture is crumbly. Sprinkle the streusel over the pears.
Place the tart on a rimmed baking sheet and bake until the top is browned and the pears are tender, 40 to 45 minutes. Serve warm.