IT's all about torque, gearing and RPM's. The wind applies a force that is trying to slow the car down. You engine produces a force called torque. The torque is applied through your gearbox to the wheels. This is called "tractive force" and it's pushing against the wind.

in 4th gear, even though you might make less torque at 5500RPM vs 4500RPM in 5th gear, the lower gear ratio gives you more force to the pavement.

Horsepower is nice number to measure to performance of the engine, but doesn't tell you the how a car will actually accelerate.

You can take 2 identical car, same weight, same engine, but give one car a 6 speed transmission, and the second a 5 speed transmission... both wiht optimised gearing, and the 6 speed will out acclerate the 5 speed almost every time... even despite having to shift one more time.