We've had ours up to "dirt-bike-heaven" Kennedy Meadows here in SoCal quite a few times. It is up into the 7-8 thousand feet elevation nose-bleeder territory. I'm pretty much ground down into 2nd gear for a loooong ways there, and eventually even into 1st in order to bend 'er around this one ridiculously tight blind right-hand corner.
My buddy has a 2003 GMC DuraMax with Allison auto. He tows a BIG toy hauler... like 35 footer. I led the way up the hill at his request (I told him I'd be draggin' my behind at a snail's pace) but he insisted. After a bit I noticed that he fell behind. I wasn't ABOUT to stop 'cause it is bloody steep! Near the top there is a gas station/convenience store. Daughter Sarah and I stopped there - where it sort of levels off and you can actually upshift again - to wait for our buddy.
Wait we did. About 45 minutes. We were kickin' it in our camping chairs sipping soda pops when John finally showed up. Turns out his (then brand-new) Duramax Allison trans overheated three times on the climb up and he was forced to pull over and let 'er cool down. Heh, heh, heh.
The 1960 Ol' Dinosaur just mosied up, cool as a cucumber, dirt-bike trailer in tow.... Ahhh... It's life's little pleasures, ain't it?