What they said, and: When I bought a new gas cap for my truck, I drilled a small hole into the plastic vent on the cap, so it will vent fumes, with no pressure build up. I believe modern gas caps (repops) won't vent out until they hit 3 psi, to protect you in the event of a tip-over. 3psi is allot of pressure in my books, so out came the little drill.
I always fill up on the way out, or just what you say happens!
