A multitude of tests are made on your vehicle to determine the appropriate tire pressure. Stopping distance is the first thing that comes to my mind. Since your front brakes give you car 80% of the stopping power and the reduction of air pressure gives the tire a broader foot print, allowing a shorter stopping distance. Again, the tire manufactures and the auto manufacturing people work very close with each other and should know best. Follow their pressure guide and get the best out of your tires and automobile!