Why should you be in the proper gear before starting down a hill in a semi-truck?

supposedly the proper gear for going down a hill in a semi is the same gear it would take to get up the hill. this came about years ago when trucks and braking systems were not as good as they are today. the reason for this being that the negine would keep you slowed enuff to control the truck if the brakes failed. remember that years ago semis did not have air brakes

In my opinion it is still safer on long steep grades to select a low gear (possibly even lower than the gear that you ascended the hill in) to decend a grade. My reasoning is this. Even with air brakes the greatest majority of class 8 trucks in North America have drum brakes that when overheated by too much use expand the drum and lessen the stopping capacity of the brakes. If you constantly using the brakes while traveling downhill you degrade your stopping power until the drums cool again and return to their original size. If you decend a grade at a speed where your Jake brakes sustain your speed and you do not have to touch the service brake pedal in an emergency situation you have the full capability of your drum brakes to slow the truck.