Ksouers has it correct. The tank is environmentaly sealed. That is why, when you loosen the cap, you get a swoosh sound. It seemed to run fine, so I don't think it affected the milage, other than loosing gas to the ground. It is close to the catylitic converter and they get hot enough to start a fire I imagine. It has been a grand truck with no problems other than routine maintenence. Timing belt at 76,000 miles and front brakes at 116,000 miles. Spark plugs at 100,000 miles, I was a little late with those and will change at next timing belt swap, around 126,000 miles. If you don't pass the environmental test, you can't get a tag until repairs are completed and you do pass.