In the first season of Grand Am competition there were five classes but by 2005 the series had reduced the number to two categories:

• Daytona Prototype (DP): This class was introduced in 2003 and was established to replace the two existing Sports Racing Prototype groups. To control costs, the cars used a tube frame chassis rather than the expensive composites used to construct most prototype race cars. Also, expenses were contained by limiting modifications and implementing rules that would be in place for five years. Approved to produce the first generation of Daytona Prototypes - for a cost of approximately $400,000 were: Riley Technologies, Doran Designs, Crawford Race Cars, Fabcar Engineering, Chase Competition Engineering, Picchio and Multimatic. Costs were also controlled by restricting the engines to production based motors supplied by Pontiac, Lexus, BMW, Porsche, Ford and Infiniti. Engine displacement was used to determine the weight of a car and whether a five or six-speed transmission could be utilized.
• Grand Touring (GT): In 2004, there were two production-based classes: Grand Touring and Super Grand Sport which initially competed in the Grand Am Cup division. These two categories were combined into a single class in 2005. Participating in this group were manufacturers such as Porsche, BMW, Chevrolet, Mazda, Ferrari and Pontiac. As with the Daytona Prototypes, engine displacement was used to establish the weight of different makes and models.

One of the top stories in 2005 was that Hoosier Racing Tire became the exclusive tire supplier for the series. Also making news was the addition of two new race tracks on the schedule. For the first time, the Grand Am series would compete at Mazda Raceway Laguna Seca and Autodromo Hermanos Rodriguez.

As in the past, the season opened in February with the Rolex 24 at Daytona International Speedway and ended in Mexico at Autodromo Hermanos Rodriguez. In between these two venues, there were events at Homestead-Miami, California Speedway, Mazda Raceway Laguna Seca, Mont-Tremblant, Watkins Glen, Barber Motorsports Park, Mid-Ohio, Phoenix International Raceway and Virginia International Raceway. There was a total of fourteen events with the series competing at Daytona twice and Watkins Glen three times.

The 43nd Rolex 24 at Daytona took place on the weekend of February 5 and 6. There was a total of 62-starters (29-Daytona Prototypes and 33-Grand Touring) which was up from previous year. Drivers would compete for 24-hours on the 12-turn, 3.56-mile track which utilized an infield road course and almost the entire tri-oval.