Stewart is the co-owner of Haas' sister team Stewart-Haas Racing in NASCAR and as a result, the Haas F1 drivers have the opportunity to drive around in the No. 14 Ford Mustang before the Grand Prix in Austin this weekend.
"A DTM car is very aerodynamic. They have a lot of downforce, while an ordinary car barely has that. So I have no idea how it will go, "Magnussen admitted when asked after the Mexican Grand Prix.
"I've never driven a NASCAR car before, but I'm pretty sure that it will be very different from the other car I drive on COTA."
The pair found it touch in Mexico, Grosjean finishing in 17th, while Magnussen did slightly better, coming through in 15th.