Grand Touring Supreme (GTS) is the name of a former classification designated to grand touring cars competing in sports car racing, originally by IMSA in the IMSA GT Championship, and later by Grand-Am in the Rolex Sports Car Series. IMSA used the class between 1992 and 1997, and Grand-Am used the class between 2001 and 2003. It was an evolution and re-branding of the former Grand Touring Over (GTO) class. Like the original GTO class, the class rules specified an engine displacement of more than 2.5 L (150 cu in), with engine design and number of cylinders being free and unrestricted. turbocharging and supercharging was allowed on engines up to a size of 6.0 L (370 cu in). Engines over 6.0 L (370 cu in) were required to be naturally-aspirated. Between 1995 and 1996 in the IMSA GT Championship, the top GTS class became known as GTS-1, while the former Grand Touring Under (GTU) class became known as GTS-2. This changed again in 1997, when the GTS-2 (former GTU) class became known as GTS-3, due to the addition of a new GTS-2 category, which allowed for existing international GT2 cars, like those used in the FIA GT Championship.^[1] IMSA also used the GTS designation for the former GT2 class cars of cars (later known as GT1) in the American Le Mans Series, between 1999 and 2004.