Pioneering a mountainous route across north-west China never previously completed by a vehicle from outside the country, the Range Rover Silk Trail expedition has successfully traversed the Xinjiang-Tibet highway, during five consecutive days and nights at altitudes of 13,000 to 17,500 feet.

On the the journey from Solihull to Mumbai, the expedition’s team members covered ground very few westerners have seen before, passing through military-controlled areas close to international borders at altitudes classified by medical practitioners and mountaineers as ‘High’, ‘Very High’, and ‘Extremely High’.

Before entering China, the Silk Trail expedition spent eight nights in scenic Kyrgyzstan at a yurt camp next to Tash Rabat, a 15th Century brick-built caravanserai that sheltered merchants travelling on the Silk Road. Next day, the expedition reached Kashgar, one of China’s westernmost and remotest of cities, where in Roman times the north and south Silk Roads split.

It was the high mountains that the three Range Rover Hybrid prototypes would have to conquer. In the next five days, heading south-east from Kashgar to Zhangmu on the China-Nepal border, the roads climbed and fell repeatedly between altitudes of 3,500 and 5,400 metres (11,500 and 17,717 feet).

At sea-level, 21 percent of the air is oxygen, but at 5,000 metres (16,400 feet) this is reduced to 10 percent. The effects of altitude were experienced by the whole team, with short bursts of supplementary oxygen required occasionally.

The performance of the Range Rover Hybrids was much less affected by the thin air. With the torque-strong alliance of electric-and-diesel engines proving only slightly less responsive than usual to throttle commands, particularly on steep inclines, the Range Rovers continued to make good progress with their heavy loads.

Auto makers rarely have a chance to test their vehicles at such extreme altitudes, so Land Rover engineers have been closely monitoring data logged about the cars’ behaviour. This demanding journey is the final validation test of Range Rover Hybrid prototypes before the model is signed-off for production.

The expedition’s in China was spent less than a mile away from the Friendship Bridge connecting China and Nepal, ahead of a crossing into the penultimate country on this epic journey to India.