Root Beer Float Cake

1 package of white cake mix (18 1/4 cups)
1 3/4 cups cold root beer, divided
1/4 cup vegetable oil
2 eggs
1 envelope whipped topping cream

In a mixing bowl, combine dry cake mix, 1 1/4 cups root beer, oil and eggs. Beat on low speed for 2 minutes or stir by hand for 3 minutes.
Pour into a greased 13 x 9 x 2-inch baking pan. Bake at 350° for 30-35 minutes or until a toothpick inserted near the center comes out clean. Cool completely on a wire rack.
In a mixing bowl, combine the whipped topping mix and remaining root beer. Beat until soft peaks form. Frost cake. Store in the refrigerator.