Yes, It’s Hot; But No, Your Gas Tank Won’t Explode If You Fill It Up
“If gasoline was truly a danger of exploding from the build-up of extremely high pressures, leaving it half-full wouldn’t be much of a countermeasure. Just as a single cube of dry ice the size of your knuckle can create a pressure bomb out of a soda bottle, a volatile liquid that could change into gas could cause a similar effect. Thankfully, gasoline reaches equilibrium at very low pressures relative to the stability of a gas tank, and both vented caps and EVAP systems are more than sufficient to maintain a constant, low pressure. The danger of spontaneous ignition is nil at temperatures achieved on Earth; if we were on Venus or Mercury, the story would be different. (But then again, so would our cars.) There are many dangers on a hot day that we should be concerned about and take precautions against. When it comes to fuel, a full gas tank poses no danger, but running out of gas in the heat can be deadly. Fill up with confidence, carry water, and be safe out there.”
There’s a fear-mongering hoax going around the internet, claiming that, on a hot day, if you fill up your gas tank, your car could be at risk of an explosion. The idea is either that the pressure will increase, causing an explosion the same way dry ice in a soda bottle explodes, or that the temperature will increase to the autoignition point, leading to a catastrophe. Either one of these could be an extremely big deal, especially if you consider how much energy is actually stored in gasoline: a full 15-gallon tank has as much energy as 860 pounds (390 kg) of TNT! But neither one of these conditions is possible on Earth at the pressures and temperatures native to our planet, even under the harshest conditions.