Preheat oven to 350°F (175°C) and grease a 9x13 inch baking pan.

In a large mixing bowl, combine flour, sugar, baking powder, baking soda, cinnamon, nutmeg, and salt.

In a separate bowl, blend together pumpkin puree, vegetable oil, eggs, and vanilla until smooth.

Gradually pour wet mixture into dry ingredients, gently folding to combine without overmixing.

Fold in chopped pecans, then pour batter into prepared baking pan.

In a medium bowl, mix flour, brown sugar, and cinnamon, then cut in cold butter until crumbly. Stir in chopped pecans.

Sprinkle streusel topping over batter and bake for 30-35 minutes or until a toothpick comes out clean.

Let cool in the pan for 10-15 minutes before serving.