Antonio Felix de Costa admitted he was surprised by the speed of BMW's DTM car after trying the M3 for the first time at Jerez.
Da Costa, who finished third in this year's Formula Renault 3.5 championship, was one eight drivers in action at the Spanish circuit during the week.
The Red Bull junior was joined by fellow debutant Jens Klingmann, as the GT regular also drove a DTM car for the first time.
Da Costa said he did not expect the DTM car to be so quick, and was delighted with the experience.
"The M3 is great fun - and is much quicker than I expected," said da Costa.
"It generates an awful lot of downforce for a car with a roof. That surprised me.
"We made progress with every stint, and I felt more and more at home. The conditions were perfect for getting to know a new car for the first time.
"The team made me feel very welcome and the engineers supported me really well. All in all, it was a really productive test."
The Portuguese driver's future plan remain uncertain, with Red Bull yet to decide his programme for next year.
He was passed over for a 2014 Toro Rosso Formula 1 seat in favour of Daniil Kvyat, but Red Bull has kept him on its books.
AUTOSPORT understands that Red Bull wants da Costa to be available for F1 simulator work on grand prix weekends, making a GP2 move difficult, and is not keen on returning him to FR3.5.