One of the secrets of travelling off the beaten track, Holtorf says, is patience.

Another is positive thinking.

All problems can be solved in time, he believes, and malaria – which he has now had eight times - is one that he reckons to overcome in two days.

“Fifty per cent of sickness is sickness you have mentally,” he says. “You get malaria and mentally you add another malaria - but I cut it by 50%. I take pills, my fever goes down, and the next day the sun is shining again, and everything is all right.”

There are some people who, faced with 99 positive things and one negative, focus on the negative thing, he suggests. “I am the other way round. If there are 99 negative and one positive, I focus on the positive.”

Holtorf and Christine never stayed in hotels. They slept in Otto, or in hammocks. Wherever they were, they shopped in local markets, and cooked their evening meal on a gas stove by Otto’s open rear door.

In almost all weather, on all continents, they sat outside.

Born in 1937 in Goettingen, Holtorf grew up in the hard years during and after World War Two. He was used to making do - as was Christine, born in 1956 in Eastern Germany. They would both scoff at younger Germans who have, by comparison, such a high standard of living but “an even higher standard of complaining”.

The tour of Africa stretched from two years to five, but it also whetted the couple’s appetite for travel, Holtorf says. At the end of 1995 Otto was put in a container and shipped to South America, a continent Holtorf knew well, having worked there for Lufthansa in the 1960s and again in the 1980s. It was natural to want to explore it again with Christine.

Otto spent most of 1996 in storage in Montevideo, but by the end of 1997 they had visited every country south of the Amazon, plus Ecuador. In January 1998, already on their 10th visit to Argentina, Otto’s odometer passed the 200,000km (124,000 miles) mark.

In the high Andes, Holtorf sometimes experienced a feeling similar to the exhilaration of the Sahara – the feeling of being absolutely alone, and “knowing that tomorrow or next week there will be nobody coming here... it really increases your heart beat”.

On one Andes crossing, on a “very windy, very dusty” slope below the San Francisco pass, at a height of about 4,000m, Holtorf had to stop and change a front wheel bearing. The job took several hours. They finally reached the pass in the late afternoon, and spent the night there, on the border between Argentina and Chile.

Later there was a moment in the mountains of Venezuela when he pressed the clutch – and nothing happened. A small O-ring had broken, but “very fortunately”, as Holtorf puts it, he had a replacement.

He carried about 400 spare parts in aluminium boxes on the roof of the car, including some in multiple examples. But thanks to a policy of preventive maintenance, which he had followed in the aviation business and applied to Otto, breakdowns were rare – and there was never one that Holtorf was unable to fix himself.

In a modern car, he insists, this would not have been the case, for one reason – computers. Otto was built in 1988, a couple of years before cars started going electronic.

“Otto is nothing but nuts and bolts that means I can unscrew the nuts and pull out bolts to repair anything that comes up myself,” he said in 2013. “In any modern car you cannot touch the brakes because it’s all electronically controlled.”

It would be simply impossible, he insists, to repeat Otto’s journey in a new car.

Holtorf also treated the car, in his words, as if it were his grandmother. He never went faster than 80km/h (50mph), and, like a lorry driver, kept the engine running when he stopped. He let some air out of the tyres before driving on a rutted road, kept the door locks greased, and if he had any suspicions about the fuel he was buying he filtered it before pouring it into the tank.

As a result, even after nearly 900,000km Otto still did not consume oil, the engine cylinders had never needed re-lining and the pistons had never been replaced. They were inspected once, in 2004, and found to be fine. After that, the engine was left undisturbed - only the gaskets were changed.

In some cases, however, roads will defeat even a car maintained in perfect condition.

In late December 1998, Holtorf and Christine decided to spend Christmas in Georgetown, the capital of Guyana, and set out from northern Brazil across more than 200km of rainforest. They expected it to be heavy going and it was, but they made it to the border, only to get well and truly stuck, on the other side, in a giant mud hole.

By now it was Christmas Eve, which Holtorf describes as a “very sentimental” day in the German calendar, often tearful in the best of circumstances. Their dinner, on this special occasion, was macaroni with tomato sauce. To cap it all Otto was on a tilt, making sleep impossible - and forall their positive thinking, the couple could not help wondering when the next vehicle would be passing through.

“It was disappointing,” Holtorf says. “Definitely not an enjoyable experience.”

After midnight, however, he heard engine noise. That had to be good news, and it grew very slowly, steadily louder. Eventually a four-wheel-drive Bedford lorry ploughed up through the mud. The crew on board could not believe what they saw, Holtorf says, but pulled them out of the hole. The next day Holtorf and Christine gave up and turned round.

Four years later, after touring Central America, the US and Canada (countries 58 and 59 on the tour), they returned. They launched themselves again into the mud of Guyana, and this time they made it.