Getting back to the definition of “mountain driving” in many areas of the east and the Ozarks of the Midwest, yes, some of the grades are 8%- 10% but they are short enough that you can almost coast up to the top, out west for example: Hwy 22 between Jackson, WY and Victor, ID is 10% for about 5mi each side with a couple of 25mph curves thrown in just for the pucker factor. In WY Hwy 14 and 14A between Lovell and Burgess JunctionI forgot the numbers but they both climb at 8%-10% for miles and miles with some hairpin curves, Hwy 16 a little south is a little better but not much. Between Laughlin and Las Vegas or between Laughlin and Kingman, AZ, I-80 between Cheyenne, WY and Laramie, I could go on and on! But there is no need of more examples.
There are a couple old RV sayings that nobody talks about anymore:
Have a small engine, go slow and get P*SS poor mileage, get a big engine go fast and get P*SS poor mileage!
There are many variables but often a small engine running at 4,000rpm at WOT (wide open throttle) won’t get any better fuel mileage than a big engine working less hard at 1/2 throttle.
The other saying is: Almost nobody complains about too much of a truck but, a lot of people complain about to little of a truck!