As just said (we also dedicated an entire article here), drag force is proportional to the square of speed and to the area of the vehicle. The larger the vehicle in front of you, the higher is the depression behind it. This depression constitutes a stonger aerodynamic toe for the car is following. For this reason, when following a large vehicle you can save fuel (to maintain the same speed the car requires less energy from the engine). On the other hand, when you decide to overtake a larger vehicle your car faces "an air wall" since you can no longer benefit of the powerful aerodynamic tow.