The 2005 NASCAR Nextel Cup Series began on Saturday, February 12. The ten race Chase for the Nextel Cup started with the Sylvania 300 on Sunday, September 18, and ended on Sunday, November 20, with the Ford 400.
Tony Stewart was the dominant driver going into the ten race "playoff" with five wins. Going into the final race before the Chase for the Cup, Stewart had amassed a 209 point lead over his nearest competitor, Greg Biffle. Points are reset going into the Chase for the cup, and Stewart’s lead was dropped to a 5 point margin. Leading into the final race before the Chase, ten racers were still mathematically contending for the final four spots in the Chase and only one point separated Jamie McMurray in 10th from Ryan Newman in 11th. In the end, the ten racers who qualified for the Chase for the Cup were:

1. Tony Stewart (3716)
2. Greg Biffle (−185)
3. Rusty Wallace (−304)
4. Jimmie Johnson (−316)
5. Kurt Busch (−412)
6. Mark Martin (−443)
7. Jeremy Mayfield (−488)
8. Carl Edwards (−602)
9. Matt Kenseth (−602)
10. Ryan Newman (−661)

Overall, Roush Racing was the dominant team going into the Chase for the Cup with five of the ten final racers from their organization (Biffle, Busch, Martin, Edwards and Kenseth).
This was the first season since his rookie season that Jeff Gordon did not finish in the top 10 in points.
The 2005 season was the first year of competition for the Dodge Charger, which replaced the Dodge Intrepid, a model that Dodge dropped from its consumer lineup for the 2005 model year. The change did little to improve Dodge's fortunes as a distant third manufacturer in NASCAR Nextel Cup behind Ford and Chevrolet. In 2004, Dodge won 4 of 36 cup races with the Intrepid. In 2005, the Charger won three races. The Charger was, however, much more of a success in the Busch Series, winning 10 of 32.
The 2005 season was the final year for the Ford Taurus, which was replaced by the Ford Fusion in the 2006 season. In addition, two prominent drivers announced that this would be their final season in NASCAR: Mark Martin and Rusty Wallace. However, Martin returned for 2006 and ran a limited schedule in 2007 and, as of 2010, plans to continue driving for Hendrick Motorsports through 2011. Also Ricky Rudd took a break after the 2005 season but returned by the 2007 Season.
One of the biggest controversies prior to the start of the season involved the elimination of two races – Darlington and North Carolina Speedway in Rockingham. The eliminated Rockingham race ends NASCAR's 38-year association with Rockingham. The races were replaced by new races at Texas and Phoenix. NASCAR also began moving start times later in the day starting this season; by 2007 the Daytona 500 finished in primetime.
Also, NASCAR announced in January 2005 that the owners of the teams in the top 35 of the previous year's standings would be guaranteed a starting spot in each of the first five races of the season. This further complicated the unique qualifying procedure for the Daytona 500, and severely affected the chances of some teams to make races after having a poor 2004. After the fifth race, the guaranteed starters were based on the current season's points, with changes possible from race to race depending on results.
The number of cars entered for each race was up slightly from the previous year. The Daytona 500 saw a dramatic increase in its car count from 45 to 56. The increase was in part attributable to a new NASCAR rule that allowed hard liquor brands to become sponsors on its race cars. Roush Racing (Crown Royal), Richard Childress Racing (Jack Daniels), and Robby Gordon Motorsports (Jim Beam) were the first to sign such deals.
At the end of the 2005 season, Chevrolet won the NASCAR Manufacturers' Championship after winning 17 events and garnering 259 points over second place Ford who captured 16 victories and 146 points. Taken from http://en.wikipedia.org/wiki/2005_NASCAR_Nextel_Cup_Series.