In motorsport it is common to have one or more test drivers who work with the mechanics to help develop the vehicle by testing new systems on the track.
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 development. Third drivers can only be used in Friday practice sessions during a Formula One Grand Prix meeting, replacing one of the team's two race drivers. With in-season testing currently heavily restricted in Formula One, this can be an opportunity to give new or younger drivers a chance to test the car. Teams have also been known to use third drivers for publicity reasons, sometimes by fielding a local driver. However, as this opportunity comes at the expense of practice time and the chance to work on car set-up for one of the race drivers, its value to the team must be considered carefully. In the 2003 Formula One season, new rules concerning third drivers were introduced, which allowed teams finishing lower than fourth in the previous season's World Constructors' Championship to run a third car and driver during Friday Practice sessions. Renault opted to take advantage of these new rules and 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 seasonMcLaren ran third drivers and also had a successful season. Despite this, not all teams opted 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. Starting with the 2007 season, the regulations were changed to their current form, so that while a team could run a third driver during Friday practice, they were no longer allowed to provide them with a dedicated third car. Subsequently, most teams choose 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, Scottish driver Paul di Resta ran in eight Friday practice sessions for Force India during 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 di Resta's case during the 2017 Hungarian Grand Prix, where he replaced Felipe Massa, who became ill during free practice.
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. ;Examples
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 with Greg Sacks at the helm. Instead of simply collecting data for the team, Sacks won the race. However, it has been alleged — reportedly admitted by DiGard crew chiefGary Nelson — that the car sneaked through inspection with an oversize engine, and thus the team cheated. 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.
has entered an R&D car sporadically since 1988 and up until the mid-1990s. 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. Earnhardt later won the championship when nine cars had retired from the race.
