Air resistance is a major force that cyclists must overcome. The bike itself accounts for 30% of the resistance felt by a rider, and the remaining 70% comes from the helmet and the rider. Aerodynamic helmets, when compared with normal helmets, can save a rider up to 1 minute over the course of a 40km ride (assuming an average speed of 50km/h) whilst specialised skin suits can save a rider up to 2 minutes under the same conditions.

Source: one