A vent won't solve the problem of overfilling. There is very little area between the top of the tank and the top of the fill neck. If you run gas up into the neck (often where the gas pump trips off), it is bound to come out when it expands.

Doing a little digging around on the net shows that gasoline expands at a rate of 0.069% per degree Farhenheit. If you pump gas out of an under ground tank that is only 60 degrees and put it into a tank that is setting in 95 degrees, it will expand roughly 2.4 percent. So if you fill your standard 18 gallon tank on a day like this it becomes a little more than 18.4 gallons. I don't think the fill neck will hold much more than a quart so there you go! Prolly all my math is wrong but we all know that it expands and runs out if you get it too full so something must be happening somewhere!