In a large bowl, combine the dry cake mix, 1 1/4 cups root beer, oil and eggs. Beat on low speed for 2 minutes or by hand for 3 minutes.
Pour into a greased 13 x 9 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 small bowl, combine the whiupped topping mix and reamining root beer. Beat until soft peaks form. Frost cake. Store in refrigerator. (I used both envelopes of Dream Whip with another 1/4 cup of root beer.)