Factual error: Nux fills the bottom of his car with petrol then lights a flare but nothing happens - the fuel should have ignited from the fumes being given off, it wouldn't need to come in contact with the liquid directly. The driver is sitting in a confined space with windows and doors shut when he lights the flare and we see the petrol sloshing about the floor of the car about an inch deep and fumes rise upwards to where the flare is.
Factual error: At the end of the sand storm scene the flare is left on the ground stationary despite being in a sand storm that has been lifting people and cars off the ground.
Answer: The truck was stuck in the sand and they had to use the tree to winch it out - the tree was used as an anchor. The weight of the truck and soft sand pulled the tree out.