Gas tanks can and will explode but many of you are correct in that Hollywood has over dramatized it. They do not lift off the ground into the air 30 feet, do a triple barrel roll and then come crashing back to the road only to explode again. Only cars load with C4 or other such explosives will do that. A car's suspension actually absorbs the bulk of the force of the explosion.
I have treated casualties of MVA's on the ground 15-20 feet away from vehicles that have 'exploded'. I can think of a couple of occasions where my back was turned away from the vehicle and I only knew the gas tank exploded was when someone mentioned it. I did not hear it over the noise of traffic and did not feel any rush of heat either.
Much like a barbeque. If you take to long to light it you get a momentary WOOOOOSH and if you are within a couple of feet you will lose your eyebrows. So too if you are within a few feet of a car when the gas tank goes up, there is a quick WOOOOSH, and you will receive some burns. Now if you happen to be standing in a puddle of gas when this happens, yes your clothing could catch fire.
If assisting in a MVA look around. Before kneeling down to help a casualty check to make sure you aren't kneeling down onto glass or in a puddle of fluid leaking from the wrecks.
Regarding comments made about persons trapped in a burning vehicle. Ultimately it boils down to doing what you can live with. But if the car is already engulfed in flames and the driver is already on fire, you are not really doing them any favors pulling them out at that point. Not only will you likely become permanently disfigured and possibiliy lose the use of your hands due to the sever burns you will suffer, but the person already on fire in the vehicle will not have much of a life after that. The nightmares of hearing these people scream are terrible. But I am comforted knowing that I am still around to have helped dozens more after that incident. If I had have sacrificed myself to extricate the woman out of the burning car, some of those people I helped later on, could have died.
You owe it to your family not to get yourself killed trying to help someone when you have neither the training or experience to do so.
Any professional first aid trainers will tell you that your safety is paramount. Please get involved, do what you can, but don't put your life at risk. We need you around to help others in future incidents. Besides if you get hurt you just cause more work for the emergency personnel. EMT's prefer to only have to deal with one pt per run.
One last comment. Small little 2 lbs (or 5 lb) fire extinguishers are useless against a car fire. Save it to use on putting out clothing that catches fire. Most fire extinguishers will only give 20-30 seconds of extinguishing agent. You can't put out much of a fire in 20 seconds. So don't even bother getting close to a car fire with an extinguisher. By the time you get close to the vehicle you will be out of agent and too damn close to the flames!
