Dunno about cars, but when I crossed the Sahara 30 years ago on a thirsty two stroke road bike I just strapped a 40 litre water container filled with petrol, on the rack on the back of the bike. Did amazingly bad things to the handling but it got me from In Salah to Agadez across the sand (no bitumen) without problem.
Now when I offroad anywhere on the bike, most recently Kazakhstan/Russia, I just scavenge as many plastic mineral water bottles out of the nearest rubbish bin, fill those with fuel, strap thme on the set, empty them into the tank as it drains and squash them flat for disposal at the next bin.
Hope this helps.
Garry from Oz.
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