The cars fuel and plastic parts.
- We don't know what's in front of the car / behind the camea.
- We don't know the wind direction during the fire.
What if it was for instance from behind the car at very high velocity as the fuel in the car burned violently and car interior, tyres working as a wick.
Well the question how fire can jump over things we already covered and all understand.
As for the "perfect circle" which isn't really a perfect circle, I don't think it's too odd. Perhaps the fire spread very quickly in one direction due to regular patches of fuel (for instance a rectangular patch of weeds)...a road could also play a part.