...no. its way more complicated than you're making it out to be. If the drag force backwards is greater than the drag force from going forwards due to the speeds involved, you're still slowing yourself down. The high speed forward here is almost certainly generating more backward drag than any gains from wind. You also have to include your relative velocity here. If you are moving forward at thirty miles an hour and the wind behind you is going at thirty five miles an hour, the effective windspeed from your reference frame is 5 miles per hour and that's the wind hitting your sail and adding speed. Meanwhile, your forward speed is much higher in your reference frame than the added windspeed. You would need wind more than twice as fast as you to gain anything even in a very simple linear model where the drag coefficients are the same in each direction. In a very simple model, wind at twice your speed would just cancel out drag due to forward momentum. And since you probably aren't moving directly with the wind behind you the entire time, the majority of that wind isn't helping and the sail is just hurting you.