IMSA GT classes are former classifications of sports prototypes in sports car racing competing in the IMSA GT Championship. The classes were used at different, overlapping times during the period from 1971 to 1998, over which the championship ran.

Grand Touring Over (GTO)^[1] is the name of a former classification designated to grand touring cars competing in the IMSA GT Championship, and later by Grand-Am in the Rolex Sports Car Series. IMSA used the class between 1971 and 1991,^[2] and Grand-Am used the class for a single season in 2000. The class had its origins in the original "TO" class used by the SCCA in the Trans-Am series, and was also similarly modeled to the FIA's Group 4 and Group 5 racing classes, but eventually evolved over time into its own category.^[3] The class specified an engine displacement of more than 2.5 L (150 cu in), with engine design and number of cylinders being 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. The original class became known as Grand Touring Supreme (GTS) in 1992.^[4]

Grand Touring Under (GTU) was a classification originally used in the IMSA GT Championship,^[5] and later by Grand-Am in the Rolex Sports Car Series.^[6] IMSA used the class between 1971 and 1994, and Grand-Am used the class for a single season in 2000.^[7] 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.^[8][9]

All American Grand Touring (AAGT)^[10] was maintained by IMSA between 1975 and 1989.^[11] These cars were All-American, V8-powered, used a maximum of 5 forward gears, and used a steel tube frame (similar to the type used in the SCCA Trans-Am Series), and were designed to compete against heavyweight European manufacturers and machinery of the time, such as Porsche and BMW, who were dominating the series at the time.^[5][6][12]

American Motors:

• AMC AMX
• AMC Javelin
• AMC Concord
• AMC Gremlin
• AMC Spirit
• AMC Hornet

Chrysler:

• Dodge Challenger
• Dodge Dart
• Dodge Demon
• Dodge Mirada
• Plymouth Barracuda
• Plymouth Duster

Ford:

• Ford Mustang
• Ford Mustang II
• Ford Pinto
• Mercury Bobcat
• Mercury Capri
• Mercury Cougar
• Shelby GT350

General Motors:

• Chevrolet Camaro
• Chevrolet Corvette
• Chevrolet Monza
• Chevrolet Nova
• Chevrolet Vega
• Oldsmobile Starfire
• Pontiac Astre
• Pontiac Sunbird
• Pontiac Firebird
• Pontiac Trans-Am
• Buick Skylark

American Challenge, also known as the All American Challenge, and formally known as the Kelly American Challenge, was a category and set of regulations for the IMSA GT Championship between 1977 and 1989. Similar to the AAGT class, these cars were All-American, use a maximum of 5 forward gears, and use a steel tube frame (similar to the type used in SCCA Trans-Am Series cars).^[13][14]

Grand Touring Experimental (GTX)^[15] is a former classification maintained by IMSA between 1977 and 1982. Some cars were based on FIA Group 5 rules; such as the Porsche 935, and some cars were based on FIA Group 6 regulations, such as the Lola T600. Like the original GTO class, the class rules specified turbocharging and supercharging was allowed on engines up to a size of 6.0 L (370 cu in); with engine design and number of cylinders being free and unrestricted. Engines over 6.0 L (370 cu in) were required to be naturally-aspirated.^{[5][6][16][17]}

• BMW 3.0 CSL
• BMW M1/C
• BMW 320i Turbo
• Busby March M1-Chevrolet^[18]
• Chevrolet Camaro
• De Tomaso Pantera
• Chevron GTP [de] (B21 with a rear hatch from a C3 Corvette for a windshield)^[18]
• DeKon Monza
• Greenwood SuperVette
• Ferrari 512 BB LM
• Kemp Mustang Cobra II
• Ford Zakspeed Mustang Turbo
• Lola T600
• SVP M81 McLaren Mustang^[18]
• Porsche 934/5
• Porsche 935
• Datsun 280ZX

Grand Touring Prototype (GTP) was maintained by IMSA between 1981 and 1993.^[5][6] The early cars were based on the almost same identical ruleset as the FIA's Group C category, but eventually evolved into a separate category. It was the successor to the Grand Touring Experimental (GTX) class, which were based on FIA Group 5 and Group 6 cars.^[17][19][9]

The IMSA GTP moniker was revived in 2023 to replace DPi as the top class of the IMSA SportsCar Championship for the merger of DPi and the World Endurance Championship Hypercar class.

In 1981, purpose-built GTP cars (Grand Touring Prototypes) appeared in the championship, and were similar to the new FIA Group C cars which would be introduced to the World Endurance Championship from 1982. The main difference between the two categories was that the former had no emphasis on fuel consumption which was highlighted by Derek Bell saying "Race fans do not come to races to watch an economy run."^[20] Brian Redman was the first GTP champion, driving a Lola T600 with a Chevrolet engine. March also fielded prototypes, with Al Holbert winning the 1983 championship with a Chevrolet powered car, changing to Porsche power later in the season; and Randy Lanier a year later with Chevrolet power. 1984 also saw the introduction of the Porsche 962, which dominated the series from '85 to '87. Nissan then took control of the series in 1988, but faced challenges from Jaguar, Porsche, and Toyota throughout the next three years. Toyota was quickest in 1992 and 1993, at the end of the GTP era, as Dan Gurney's All American Racers team campaigned the Eagle Mk III, a car so dominant that it has been blamed for the demise of the class. Along with the GTP cars, the Camel Lights cars, a smaller capacity, non-turbocharged, lower powered prototype category was introduced in 1985. Argo Racing Cars was the first 'Lights' Champions, followed by Spice Engineering. Other well known participants were the Tiga, Royale, Alba, Fabcar, and Kudzu.

Starting with the 1986 season, the GTP category had their own decal, which was similar to the IMSA GT side decal, with a P being added to denote their category.^[21] Camel Lights cars also used the same decal^[22]

There were many other manufacturers in the GTP class, such as URD Rennsport, Spice, Intrepid or Gebhardt, and in the early 1990s, Mazda.

Following a successful heart surgery in 1987, Bishop began to rethink his priorities. He was approached by Mike Cone and Jeff Parker, owners of Tampa Race Circuit. In January 1989, Bishop and France sold the series to Cone and Parker. The new owners relocated the IMSA headquarters from Connecticut to Tampa Bay.^[20] Bishop would stand down as president in favor of Mark Raffauf, who was his deputy, and its representative on the ACCUS board.^[23] Cone and Parker sold it to businessman Charles Slater. Both lost millions attempting to revive the sagging TV ratings.^[20]

By 1992, there were a number of factors that led to the decline of the GTP category. Porsche concentrated on its IndyCar(Porsche 2708)program when critics stated that the Zuffenhausen marque should have built a followup to its 962.^[20] Back in 1988, Al Holbert realized that the 962 was beginning to feel dated. He proposed a follow-up open-top Porsche powered racer which would also be sold to customer teams. That project never got off the ground due to Holbert's death in an aircraft accident later in the year.^[20] For some, much of the blame was on the organization for allowing the Japanese "works" teams to dominate the series. Under Bishop's original vision, privateers and "works teams" were able to race equally. Privateer teams walked away, while the Japanese economy started to go downhill. These factors led Nissan and Mazda to leave the series. Critics predicted that the decreased variety of cars would disappoint race fans, and in fact, it did finally kill the series in 1993.^[20] GTP cars ran their last race on October 2, 1993, at Phoenix International Raceway.^[24]

The GTP category was credited for many innovations in the U.S., including antilock brakes, traction control, and active suspension.^[20] Dave Cowart and Kemper Miller's Red Lobster sponsored team of the early 1980s would innovate race team hospitality, practices which were subsequently adopted by virtually every other team.^[20] For those that competed, GTP was recognized for its camaraderie among drivers, especially rivals. But Hans Stuck, commenting in the foreword of the book "Prototypes: The History of the IMSA GTP Series", sarcastically compared the series' camaraderie to Formula One's lack of such.^[20]

• 
  Eagle HF-98
• 
  Jaguar XJR-9
• 
  Jaguar XJR-9
• 
  Eagle Toyota HF-89
• 
  IMSA GTP "EAGLE" #99
• 
  March-BMW IMSA GTP (1986)
• 
  Chevrolet Corvette GTP.

Grand Touring Supreme (GTS) is a former classification originally by IMSA 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 GT3, 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.^[7] 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.