I don't travel over very many highways with long steep down hill grades other than the Kootenay pass in BC. It is long .... 19 miles long. The average grade over that 19 miles is just under 4% but there are very short sections that hit over 13%. Towards the top of the pass is a 2.5 mile section that averages 7.7% grade. I've descended that grade a number of times at 21 - 25k lbs gross with various trucks. Even my '03 Duramax at 21k lbs gross and no exhaust brake held back at about 40 mph and 5100 rpm without using brakes. If you go slow enough you don't need an exhaust brake. :B
