I've got one on my 95 Nitro, I take the cap off when I'm fueling, it vents the tank while fueling and makes it a lot faster w/ less blow back. I just make sure to put the cap in my pocket when I take it off or I'm prone to leaving things on the deck and driving off.........:wacko:
Fuel fill vents as stated. Older models didnt have the vent incorporated into the fuel filler neck. If it helps to unscrew them while filling, just dont forget to put them back on.