Third driver

In motorsport it is common to have one or more test drivers that work with the mechanics to help develop the vehicle by testing new systems on the track.

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 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.

However, it has been alleged — reportedly admitted by DiGard crew chief Gary 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.

Richard Childress Racing 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 (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 development.

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Wikipedia