The socalled Lagunas route through highland desert in Bolivia is around 450 kms from Uyuni to San Pedro de Atacama. But there is actually as gas station in San Cristobal around 90 kms out of Uyuni so the stretch from there to San Pedro de Atacama is approx 360 kms depending what actual route you end up doing through the Lagunas area. But of course at that route you will ride in 1th and 2th and 3th gear a lot and fuel milage wont be good.

Also as mentioned above - south in Patagonia gas stations is often out of fuel - so always fill up where its possible.

Also in Brasil at the R319 Transamazonia there are some long stretches inbetween gas stations.

So what range is needed? Well it depends, where you plan to ride. Very few ride the Transamazonia, not so many do the Lagunas route. The best answer is the longer range you have the better it is. I wouldnt go anywhere unless I had a good 400 km/250 mile range.

Fuel quality - only place with low quality fuel that I encountered was Bolivia. They sell only 85 octane RON (used in Europe and most of the world) which is around 81 octane AKI (used in Usa etc). Even riding a bike with relatively liw compression one felt this, and one had to be a bit careful with the throttle.
I also experienced low fuel milage in Brasil - they seem to put a lot of «other stuff» in their petrol. Didnt hurt the engine though….
Didnt go to Venezuela so cannot say anything about fuel there.

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In the end everything will be fine. If its not fine its not the end....