Result of downshifting to first gear at high speed - GIF

[Frank S]

Dropping a driveshaft or a transmission grenade on you on a steep down grade is one thing you only want to experience once. You might not survive the next time.
Big baldy also known as Cabbage pass in Eastern Oregon used and may still have signs posted on the down grade stating what gear and speed was the maximum safe speed for the number of axles and weight you had. Bigger engines with jake brakes and 18 speed or auto shift transmissions and better brakes have done a lot to alleviate downhill dangers, but you still don't want to go down Black Mountain at 60 MPH with a load of slinky. Unless you are as good as Sandra Bullock and can drive with half your wheels in the air.