Just got back from Hawaii where I drove up to the summit of the Mauna Kea volcano. At it highest point the road is almost 14,000’, but the truck engine seemed to run fine. I would have thought it might be staved for oxygen (my lungs were) and not run smoothly.
I know there are higher roads. How high would a road have to be before a vehicle’s engine would start having problems?
At higher altitudes, the engine will still run smoothly because the computer compensates for fewer air molecules entering the engine by injecting less fuel. Even back in the carburetor days, the fuel was mainly metered by the airflow passing the jets and so altitude wasn’t a big issue, although sometimes cars that lived at high altitudes would require high altitude tuning kits to meet emissions standards.
However, as the air gets thinner, the effective compression ratio of the engine goes down (since it’s squashing fewer air and fuel molecules), and you lose power. In theory, once you get high enough the engine won’t produce enough power to keep itself running and will stall, but that doesn’t happen at any driveable altitudes.
You don’t really notice altitude when you’re driving as much these days simply because cars are so much more powerful than they once were. When people were driving 36-HP VW’s and two-ton land barges with wheezy 6-cylinders, you really noticed when you started losing those horses as you climbed, but in a modern car unless you’re on a race track you’ve got quite a lot of power to spare. The power loss is also somewhat offset by lower drag when you’re actually driving.
There was an episode of Top Gear where they bought old, cheap SUVs and drove from the Amazon basin to the Pacific. There were two routes across the Andes. They tried the shorter, higher-altitude one first. I think they got over 17,000 feet before they gave up and took the other road. The hosts and the vehicles were giving out in about equal measure, and the were clapped-out junkers. (And the vehicles were pretty old, too.)
Actually my first question is if the truck was a diesel or not…and of course a turbo…by their nature of operation diesels can operate in a very wide range of altitudes. That said a modern gasoline engine is well equipped to handle variations in altitude, the general goal is up to 3000m, thanks to computer control.
Most modern cars with Oxygen sensors will do fine.
Turbo engines will be less influenced once the turbo is a max boost, assuming waste gate is based [really] upon boost pressure and not some ECU hard coded RPM–which some are. (Hey, its cheaper, that’s why!)
The equation for power loss is:
HP Loss = [Elevation (ft) * 0.03 * sea level HP]/1000
IOW: 30% loss at 10K ft.
I had a Honda CX500, not a seriously underpowered bike. I did notice that at one time going over mountains, it did run ok until I needed more power. I realized I had the throttle a lot further back than normal for climb, and when I “floored” it, at full throttle, it did not have the oomph it had at normal altitude.
I used to have a CX500. 
In older cars, I’ve wished there was a mixture control like on an airplane.
I used to have a Datsun Nissan Sentra Diesel. It was rated at 47 HP at sea level (and got over 40 mpg city). The thing wasn’t worth a crap at altitude- I’d have to go clear down to second gear to get over the climb at Monteagle in Tennessee. It was also way down on power driving around Dahlonega, Georgia.
Years ago, I drove my VW bug from the Midwest to California and back a few times. I found it useful to get it adjusted for altitude once I arrived in the Rockies, then readjusted when I got back to the flatlands. I don’t know what the adjustment was, but it made a noticeable difference on such a small engine.
I drove (or rather, was driven) around Tibet at heights up to 5200 meters, without any apparent trouble.