Most aluminum and poly tanks I've seen were about 1/8" thick. If you consider an 1/8" sheet of aluminum with bracing (baffles) with the short dimensions of a fuel tank, that 1/8" aluminum is pretty tough. I've sen aluminum tanks with pretty deep gouges in them that never leaked a drop. There is a better chance the aluminum tank will crack from road vibrations long before any other issues will likely happen, and that event is several years down the rough roads. The biggest issue with the poly tanks is also cracking from vibration.

As far as melting a fuel tank, if the fire gets hot enough to melt a fuel tank, everything else would be toast by then, the melting tank would be a mute point. Consider the fact that before the aluminum can melt, it has to reach 800 degrees. That would mean everything surrounding the tank also has to be very close to that 800 degrees, including the air inside and outside the tank. The fuel inside the tank also has to reach 800 degrees before the aluminum can melt. Any material or plastic in the car would be burned up long before that happens.

That said, when
I build my tanks, I build them out of 14 gauge steel. I'm cheap and aluminum is expensive. Gene