It depends how fast the planet depopulated. In Mad Max the original problem was water shortages. If most of the population was wiped out in the water wars, it’s reasonable that the dregs of fuel in the system could last the few survivors a long time.
Is it more efficient (and/or easier) to make ethanol to fuel an internal combustion engine, or instead to burn the fuel you would have used to distil the ethanol to fuel an external combustion engine? (i.e. a steam engine.)