Well, this is only vaguely aviation related, but anyway, maybe the car-savvy members here have an answer. We had rented this little car in mountainous northern Argentina:
It happily climbed to more than 14000ft without a significant loss of power. I took a look under the hood to search for the turbo but I couldn’t find it. The only thing I saw was a lambda sensor in the exhaust manifold.
This is not possible, of course. Explanations I could think of are:
There might be other explanations, but they really start to be far-fetched…
Very often rental cars have special firmware to reduce the power.
The highest I’ve driven a car was about 11,000 ft, a normally aspirated Mazda MX-5 that was very low on power at that point, so I couldn’t explain why your rental car didn’t suffer similarly. It looks to be a Latin American GM of some kind, and I don’t believe they are turbocharged (?) An interesting mystery 
An engine does not lose power at altitude. It loses MAX power, so unless you use more than 60pc power on a regular basis, you would not notice.
Also, a car does not need power to maintain altitude, the road does that for it, so the power requirement for a climb is lower.
So all in all it is quite difficult to tell if a car actually loses “significant” power unless you actually need that much power, for example when towing a trailer.
Rule of thumb apparently is that a car loses 3-4% power per 1000 ft, so that would mean that your engine would suffer anywhere between 42-56% power loss.
I would suggest a sixth option: you didn’t know the car very well to notice the reduced performance.
Other than that I think it’s highly unlikely you would miss such a massive reduction in power loss. It would be a Golf III 1.9 TDi transformed into an 80’s Fiat Panda.
Archie wrote:
It would be a Golf III 1.9 TDi transformed into an 80’s Fiat Panda.
Except that the Golf III TDi has a turbo.
Archie wrote:
Rule of thumb apparently is that a car loses 3-4% power per 1000 ft,That sounds very reasonable given that air pressure drops by about 3% per 1000 ft (at altitudes where you would typically drive a car).
Where did you rent it from? On the Bolivian Altiplano for example, the cars are set up to operate at high altitudes. May well be the same here.
I can make the aviation link for you:
A lot of the Mercedes cars I work on are 1960s with mechanical fuel injection. The story goes (and I have no wish to disprove it) that Me109s had Mercedes engines and they used mechanical fuel injection that had barometric compensation and that same system can be found on the cars that I work on…….
Plausible that modern cars have a software equivalent.
As well as altitude you also need to consider temperature….
On an ambitious drive from Portugal to Granada in my R4 (dont laugh please) I learnt all there was to know about density altitude…passing 4200ft on a hill with an OAT of 42c
I could not get out of 1st gear until it levelled off …. fortunately it was all downhill after that
