This study investigates the air resistance due to deflector configurations of commonly used largetrucksat a driving speedof 80 km/h. Of the 6 models that are considered, the drag causedby the pressure on the front part of the trailer is largest for model 1. For model 1, the lift caused by the difference between the velocities of the upper and lower streams isgreatest amongall the models. Model 6 has the least resistance against the flow stream consideringthedrags and lifts of theflow models. Anoptimal design can be obtainedby investigating thesimulation analysis ofvarious deflector configurationsfor the upper part of largetrucks; this optimum deflector configuration will help reduce the air resistanceon large trucks.