Root Bear Float Cake

A friend sent me this recipe, but have never made it

1 package white cake mix
1 3/4 cups cold root beer, divided
1/4 c vegetable oil
2 eggs
1 envelope of whipped topping mix

In a mixing bowl, combine cake mix, 1 1/4 cups rootbeer, oil and eggs. Beat on low speed for 2 minutes or stir by hand for 3 minutes. Pour into a greased 13x9 baking pan. Bake at 350 for 30-35 minutes or until a toothpick inserted near the center comesout clean. cool completely on a wire rack.
in a mixing bow, combine the whipped topping and remaining root beer. beat until soft peaks form. frost cake. store in fridge.