"HUH??? Have you driven I-17 from Phoenix to Flagstaff or Flagstaff to Phoenix??? I've been down to 15mph or lower behind heavy trucks when passing was impossible on that route. If you can live with the reduced speed on a hard climb and your engine and transmission is not overheating, and you're running within weight limits, YOU ARE NOT OVERLOADED AND YOUR TOAD IS JUST FINE. You do not have to fly up steep hills at 70mph.
Frankly, someone having fun following me up a steep hill does not concern me at all. I will be in the right lane, obeying the local and state laws. and if my speed bothers you, go around. My job is not to keep someone behind me having fun."
I have driven those routes dozens of times and have NEVER seen anyone down to 15 mph. I do see trucks chugging along at 35 with their flashers on and yes passing them is a PITA. Driving far slower than the flow of traffic is not only dangerous, rude and foolish it is also illegal. No one said you have to "fly up steep hills at 70".
If you are going 15 - 25 you are not obeying the local and state laws or the feds for that matter and the interstates are their jurisdiction. I stand by my opinion that if you are only able to do 25 mph on a 6 percent grade something is wrong. You are correct that your job is not to keep someone behind you having fun...Your job is to drive safely. Sorry you don 't get that.
As always... opinions and YMMV.
:R