gas tank vapor pressure

Depends on the car and the fuel injection system it has. Tanks need to be vented, but the emissions system will close the vent and pull vapors from the charcoal canister at times. Vapors expand on hot days, as well- requiring the venting. Removing an emissions system is extremely illegal, but if you're building a race car or vintage car you may not need to retain all the equipment but there's rarely a good reason to.

Do a "google" search for gasoline vapor pressure. That will help you understand vapor pressure and how it varies summer to winter.