Agreed on the petrol - The risk of starting while in a car is very low (at least a modern one), but the risk if you have a fire from another cause is certainly higher. 50 gallons of gasoline is lot of stored energy (as is a couple of propane tanks). All of this risk is still very low, and I would be most concerned with something that can start a fire. Of course some electric cars like the Bolt have been problematic.