Looks like a ZR spoiler
Actually this is not the world's fastest fire engine. It is a 1999 Rover 75 Connoisseur (S967 YOL) built as part of the Surefire 1 project to provide fire cover for land speed record attempts. It was the support vehicle for the fire chase vehicle shown below and carried fire extinguishers, emergency glass-hammer and a computer system with dash-mounted touch screen for accessing details of special fire risks.