u/Max_Rockatanski

The 2nd film literally explains it right at the start:
"Only those mobile enough to scavenge, brutal enough to pillage would survive".

Yeah they're burning through fuel and build insane cars which seems counter intuitive, but in reality - that's what ensures their survival. Driving a slow fuel efficient 4 banger minivan makes you an easy target. MM2 literally shows you wreckages of slow and unmodified cars run down and burning right in the middle of the road right at the start of the film. I dunno, I thought ppl would take the clue but it keeps coming up like some kind of silly plot hole. It ain't.
You just have to burn through whatever fuel is left if you want to survive out there or find a way to hide, escape or withstand inevitable attacks.

The 'trade negotiations' scene is awesome. Plain and simple. Because if you pay close attention you'll notice that Dementus didn't win anything, but instead he was played like a fiddle.

Dementus wanted double of everything. Instead he agreed to 33% more water and 25% more potatoes only. He was low balled and he still agreed. Why?

Because the Immortan realised that Dementus is full of it, seconds after Furiosa said she wasn't his daughter. He realised that he is lying about rigging Gastown as well. It comes up again later in the film when Dementus fakes blowing up Gastown and Immortan knows there's no advantage to it, so he fooled Dementus using his own tactics (fake Gastown smoke for fake armada dust cloud).

But here's the best part: Immortan literally hired Dementus as security for his War Rigs and Gastown for pennies. (Because that place, along with the Bullet Farm, did not have their own armies yet - they were built after Dementus was killed).

Dementus was technically allowed to 'run Gastown' and the moment he ran that place to the ground and attacked the Bullet Farm, the Immortan finished him off. So the Immortan didn't really give Dementus much control to begin with, he gave him an illusion of it and since Dementus was too narcissistic and stupid - he didn't even realise it.