Preheat the oven to 375°F and lightly grease a standard 12-cup muffin tin or line them with muffin cups.
To make the muffin batter, whisk together the flour, granulated sugar, brown sugar, baking powder, cinnamon, nutmeg, and salt.
In a separate bowl, stir together the melted butter, oil, eggs, vanilla, and sour cream until creamy, then add the wet ingredients to the dry ingredients, stirring gently until well combined and a thick batter is formed. Fold in the peaches.
To make the streusel, stir together the granulated sugar, flour, cinnamon, salt, and nutmeg until well combined. Stir in the butter and pecans until everything well combined.
Fill the muffin cups 3/4 full with the batter, then evenly top with streusel. Bake for 20-25 minutes or until lightly browned on top and when a knife inserted into the muffin comes out clean. Serve warm.