Test 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 specific motorsports
NASCAR
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.
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.