Adding this insert kit into a '65-73 Mustang or '67-70 Cougar fuel filler neck reduces / eliminates fuel overflow - spillage while filling up with gas using modern unleaded fuel nozzles. With the wide open neck on a classic Mustang, unleaded nozzles don’t shut off like they do on a modern car (or like old leaded fuel nozzles did) so you can get Splash-Out every time you fill up. This mod fixes that, and it also helps hold the nozzle in place so you might be able to pump hands free.