In specific motorsports
In NASCAR, test driving has mainly related to "Research and Development cars". A team might hire a driver and put him in the race to gather more data. NASCAR teams rarely have specific test drivers on staff.
In 1985, DiGard had Bobby Allison battling for the championship. For the Firecracker 400 at Daytona, DiGard set up and raced what is called a Research & Development car (a one-off car entered to a race primarily for team improvement) with Greg Sacks at the helm.
Instead of simply collecting data for the team, Sacks won the race.
The impact of the R&D car was significant: Reportedly angered that the team was focusing its attention elsewhere, Bobby Allison left the team mid-season — two weeks after Sacks' July 4 win. Sacks was hired to race for the rest of the year, but never captured another Top-5 finish in 1985.
In 1993, an RCR car intended as an "R&D car" driven by Neil Bonnett was entered into the last race of the season at Atlanta, and promptly dropped out of the race immediately after the first laps, finishing 42nd. It was extra security for Dale Earnhardt to win the championship; he needed to finish above 34th spot to defeat title rival Rusty Wallace. Dale later won the championship when nine (of 42) cars had retired from the race.
In Formula One, the term third driver is used to designate a test driver. Third drivers do not compete in Grands Prix, but are used by teams to help the race drivers and engineers with car set ups.
Third drivers can only be used in Friday practice sessions during a Formula One Grand Prix meeting. Teams that finished in the top four of the previous season's World Constructors' Championship are not allowed to have third drivers.
Having a third driver is usually seen as an advantage to a team. In the 2003 Formula One season (when the third driver rule came into effect, albeit in a different form), Renault opted to take advantage of new rules that allowed them to take part in an extra Friday session with three cars. Renault subsequently had a successful season. Similarly, in the 2004 Formula One season, BAR ran third driver Anthony Davidson and finished second in the World Constructors' Championship. In the 2005 Formula One season McLaren ran third drivers and also had a successful season.
Despite this, not all teams opt to take advantage of the third driver rule, with some teams saying that it was too expensive to run a third car on a Friday.
Third drivers are not always used for performance reasons either. Teams have been known to use third drivers for publicity reasons, sometimes by fielding a local driver. Third cars are also allowed to run in different liveries to the race cars.
Starting with the 2007 season, the regulations were changed so that while a team could run a third driver during Friday practice, they were only allowed to run two cars. This meant that any track time the third driver had was at the expense of one of the teams race drivers, because of this most teams chose not to run a third driver, in order to give their race drivers maximum practice time. Third drivers are still used occasionally, most notably Sebastian Vettel drove for the BMW Sauber team on Fridays at the start of the 2007 season, scot Paul di Resta is scheduled to run Friday practice for the Force India team at selected Grands Prix in the 2010 season.
Also, on several occasions third drivers have acted as reserves, racing if one of the team's two race drivers is injured or otherwise unable to race, as in Bruno Senna's case in the Renault F1 team in the 2011 season.