Prototype 900 Class

The Prototype 900 class is comprised of two divisions; Le Mans Prototype (LMP) and the Grand Touring Prototypes (GTP). The two types of cars are classified together in ALMS races. LMP- Closed-wheel, purpose-built racing chassis. Limited to 90 litre fuel tanks, six forward gears, 6.0 litre normally aspirated or 4.0 litre turbocharged engines, carbon fiber brake discs and 16 inch wheels. The cars, except in specific cases, may weigh no less than 900 kilograms.  Includes cars such as the Panoz LMP 07 and LMP-1, Riley & Scott Mk3, Lola B2K, Audi R8 and Cadillac Northstar LMP.

GTP- Low-volume, production-based, two-wheel drive cars formerly known as the "GT1" class. Limited to 90 litre fuel tanks, five or six forward gears, carbon fiber brake discs and 14 inch wheels. The cars, except in specific cases, may weigh no less than 900 kilograms. Unibody constructed cars are permitted to compete with normally aspirated engines up to 8.0 litres in displacement and turbocharged engines up to 4.0 litres. Though popular in the past, closed-roof Prototypes are rare now. Bentley ran a new GTP car last year at Le Mans and a Lotis Elise is expected for some ALMS events this year.

Prototype 675 (LMP 675) Class

The LMP 675 class, featuring lighter and less-powerful Prototypes, was new to the ALMS in 2001. The cars cannot weigh less than 675 kg, but are similar to LMP 900 cars in most specifications. Engines are limited to 3.4 litre (normally aspirated) and 2.0 litre (turbocharged). The cars use carbon fiber brake discs that can be slightly smaller than LMP 900 cars, and 14-inch wheels. Lola and Reynard cars are common in this class, as is the Pilbeam.  AER MG, Nissan and Judd have designed engines for the LMP 675 class.

Grand Touring Sport (GTS) Class

The GTS class is comprised of production-based, two-wheel drive open or closed cockpit cars. Limited to 100 litre fuel tanks, six forward gears, 8.0 litre normally aspirated and 4.0 litre turbocharged engines, and 14 inch wheels. Carbon fiber brake discs are permitted. Wings and additional approved aerodynamic devices may be added to the body. The cars, except in specific cases, may weigh no less than 1100 kilograms. Includes cars such as the Dodge Viper GTS-R/T, Ferrari 550 Maranello, Chevrolet Corvette C5-R and Saleen S7R.

Grand Touring (GT) Class

The GT class is comprised of production-based, two-wheel drive open or closed cockpit cars. Limited to 100 litre fuel tanks, six forward gears and 14 inch wheels. Carbon fiber brake discs are NOT permitted. Wings and additional approved aerodynamic devices may be added to the body. The cars, except in specific cases, may weigh no less than 1100 kilograms. Cars are allowed normally aspirated engines up to 8.0 litres or, utilizing turbocharging, up to 4.0 litres. Includes cars such as the BMW M3, Callaway C-12R, Chevrolet Corvette C5-R, Ferrari 360 Modena and Porsche 911 GT3-RS.

Each class has its own pole winner for every race, along with a driver who sets the fastest lap for each class in every race. The top three race finishers in each class comprise the "podium" for that class.  One of the most exciting elements of ALMS events is the speed differential between the Prototype and GT cars. The races feature a great deal of passing and dicing as drivers compete for their own class wins while maneuvering in difficult traffic situations around road racing circuits.