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It has been suggested that this article be merged with Grand Touring Experimental, All American Grand Touring, Grand Touring Over, IMSA GTP, Grand Touring Supreme, American Challenge and GT3 (1998–1999) to List of IMSA GT classes. (Discuss) Proposed since December 2023.

Grand Touring Under (GTU) is the name of a former classification designated to grand touring cars competing in sports car racing, originally used by IMSA in the IMSA GT Championship,^[1] and later by Grand-Am in the Rolex Sports Car Series.^[2] IMSA used the class between 1971 and 1994, and Grand-Am used the class for a single season in 2000.^[3] The original class rules specified an engine displacement of under 2.5 L (150 cu in), natural aspiration, and was the secondary class below the top tier class, GTO. The class later became known as GTS-2 in 1995, then GTS-3 between 1996 and 1997, and eventually evolved into the GT3 class for 1998.^[4][5]

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