Way to much Hollyweird.

Now if you have a rupture tank. The spilled fuel evaporates enough to for a cloud. Then you get an ignition. You will have a pretty hefty WHUMP and a respectable fireball.

But if it is already burning you don't get the mixture for an explosion. You might get a flare up but not much more. Gas tanks have vents to prevent a pressure build up. Aslso the fuel ines ill vent some pressure.

Now that is not to say you should cozy up to a burning car. If the fuel tank is not already burning if it does light the flare up can spread burning fuel faster than you can get away.

The danger of an explosion would be more likely from something stored in a vehicle. A propane torch or a propane tank for a grill could give you a big surprise. You don't know what else is in the car.

If you can't help Why stand around. Get on with your own business. Let those with the needed skills and equipment deal with it.

OTOH if you can help get others away from the fire and keep it from spreading. Ifyou have gear such as an extinguisher realize its limitations. If the fire is small you might be able to put it out. If it is fully involved and a total loss anyway why waste you gear. The only thing you can do then is to keep it from spreading.