I fitted sill tanks to my rangie last year in preparation for the trip. Gave me an onboard capacity of 160l then also had 2 10l jerry cans on the roof rack. As my rangie is efi we connected the tanks so that they fill into the main tank. With this arrangement we didn't have any issues between petrol stations. There are actually lots of them up there. I also fitted a snorkel, more of piece for mind as we weren't too sure how deep the crossing were going to be.
The rangie will handle it very well.
Have a look through the trip reports as I have made other comments in them.
Both sill tanks and snorkels appear in the markets from time to time.
98 Harvey the tractor - 300 tdi Defender Wagon
84 Alfetta GTV
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