The KKH may be the highest PAVED road in the world, but there are GRAVEL roads you can drive that are even higher.
https://www.dangerousroads.org/rankings23/5269-highest-mountain-passes-of-south-america.html
From a gasoline engine perspective, I wonder what's the highest altitude you would expect a non-supercharged car or truck to be functional at?
1: Engine vacuum is said to decrease by 1" for every 1,000 foot of altitude. I assume that at some point, there won't be enough vacuum for the engine to run.
2: With supercharged engines, I guess you could start it at a lower altitude and it would still be running at the high altitudes. But if it was turned off at the higher altitudes, I wonder if it would restart?
3: Finally, I wonder at what point the ECU would go: "And the air density is . . . Oh, hell no, that CAN'T be right!" and shut down?
Also, if you have a vacuum power booster as opposed to a hydroboost system on your brakes, I imagine coming back down from such heights would be interesting . . .