Once out of Europe and into Iran, yes, we used the spare fuel tank. At the time it was a complete unknown on what lay ahead, not only fuel supply but the political situation. We arrived in Tehran 3 weeks after the American Embassy was seized. The goat tracks in the Turkish mountains severely reduced the tread on the rear tyre so with the help of a friendly carpet seller we eventually purchased a dirt tyre as a just in case. The desert in central Iran started off with a fantastic American built Highway that fairly soon was blocked off with piles of sand, the Americans along with many other expats had left the country. The track was pretty dire all the way into Baluchistan and finally to Zahadan................Did we use the spare tank again, no, not for fuel anyway, just a luggage rack