will modern-day gas tanks explode if filled too high

This trope also has some overlap with Bullet Sparks see that page for the fairly limited number of ways this can be a Justified Trope. For vehicles that explode on any kind of incidental impact, see Every Car Is a Pinto. This is about deliberately shooting a fuel tank. Of course, the behavior of the vehicle during the explosion is still the same, aside from getting quite a significant jolt from the impact of a 20mm rifle round traveling at 800m/s (similar to the muzzle velocities of some naval cannons).

It is important to note that, while conventional bullets or even tracer bullets do not often generate the heat required or have enough surface area to diffuse this heat fast enough to ignite fuels like diesel or jet fuel, white phosphorus rounds, like those shot from the Lahti L-39 Finnish 20mm Anti-tank/Anti-aircraft sniper rifle (more commonly known as the 'Elephant Gun') or the minigun wielded by the Mythbusters, are more than capable of both penetrating the outer shell and fuel-tank wall of the vehicle they are hitting and delivering enough heat to cause an explosion. Avgas (aviation gasoline, the super-high-octane blend used in piston-engined aircraft) can blow up like this (with a very Hollywoodish fireball), which is why airplanes since the 1920s have things like self-sealing fuel tanks and inert-gas-pressurization for their tanks. Jet fuel's extremely-high ignition temperature causes it to behave a lot like diesel. Diesel pretty much won't blow at all unless you light a fire under it and keep it burning until it causes a BLEVE (Boiling Liquid Expanding Vapor Explosion), which will take a while to accomplish. An ordinary passenger car's gas tank with unleaded gasoline will produce a 'POP' and a quick (and anticlimactic) flash of flames underneath the vehicle with all four tires remaining firmly planted on the ground, though the resulting fire may consume the vehicle pretty quickly.

Even if (against all odds, logic, and reason) the fuel tank does explode, the result is rarely the spectacular car-flipping fireball that Hollywood promises. Besides, if you have a minigun then you're not going to need to shoot the fuel tank anyway. A real-life fuel tank explosion was only possible when they used a minigun armed with incendiary rounds which, suffice to say, is generally unlikely. This myth was busted by MythBusters, as they proved that in most cases it simply will not work. This makes for an effective tactic in battle, as it's filled with nitro, in addition to the gas.

In fiction, shooting a fuel tank makes it explode.

Will modern-day gas tanks explode if filled too high