Depends . . . on LR's with an ECU, it will be monitoring the air/fuel mix ratio and also reducing the amount of fuel to attempt compensate for the reducing 'density of air'
The actual amount of Oxygen in air ALWAYS remains proportionally the same at approx 21% . . it's the atmospheric pressure which drops, this is not normally a problem below about 8,000ft, above that altitude and you would suffering from Altitude sickness . . a blinding headache initially, which would make driving difficult !!!
Last year in the French Alps, I drove my Disco up to an altitude of just over 2,000 metres (6,550ft) and was also towing me van, and didn't really notice a loss of performance . . . so the ECU etc was obviously doing a good job !!!