I use animation curve v2, but if you click on the green curve, a window pops up. You will have presets at the bottom to choose 1.
I'. assuming your speed is 1 that you want to go down to 0. Just select a new preset or click on the handles, drag 1 side up to 1 and the other side down to 0. The animation curve is set up for 1 sec (left to right) and from 0 to 1 (bottom to top). just move the left upwards to 1 and the right one down to 0.