There are 15%grades in Yellowstone. 18%grade in the front of my sister's house. A few local boat Ramos are that steep. Assuming these are what you may be going up or down, do you want bare minimum, a bit steeper you literally stall out? Or some percentage higher? Hence my 30%grade number.
Altho I did out in a set of stairs along a driveway a few years back. 6" rise, 18" run, so 33%grade for over 100'! I know of a block on Queen Anne avenue on north side that is 25%.
IF you have any chance as I do to drive around on these steeper grades, the min 12% grade that current spec calls for is 200% of max freeway grade. Hence why some have to go into 4lo to move trailer. They would could not do it, ie stalked out their rig in red! I stalked out my dmax once. Glad I had 4lo, pulling 12k equipment!
Marty
