Fernando Alonso says he was "very far away" from finding a good lap in his NASCAR test run in Bahrain.
While seven-time NASCAR Cup champion Jimmie Johnson impressed in a Formula 1 car during a swap on Monday organised by McLaren, Alonso said car characteristics and a lack of tyre grip left him struggling to find the limit in Johnson's Chevrolet.
"I think I was very far away from a very good lap," explained Alonso.
"Driving style [wise], it was still not very clear to me what the best way to perform a lap is.
"I found a lot of problems on braking. The car has very poor retardation because of the weight and the steel brakes. They behave very differently compared to the ones I'm used to.
"And traction, [on] these tyres with the amount of power those cars have, is very difficult to manage. So in first, second [and] third gear you're still spinning the tyres.
"I didn't know if it was better to go full-throttle and spin the tyres and really move forward or control with the throttle and maybe lose a little bit of performance on exit.
"It's not so clear for me even after one day what is necessary on these cars."
Johnson said the Bahrain circuit was not a suitable place for Alonso to have his first taste of NASCAR in order to understand the car's performance.
"He [Alonso] didn't get a fair shake of experiences," said Johnson.
"Our cars are heavy and have a high roll centre, so when you put them on a banked track they have a chance to shine.
"Dover, Bristol, even some of the banked 1.5-mile [ovals], would really impress him."
Alonso said the test left him uncertain about how it would feel to drive on an oval, especially if there were other cars around him.
"I told Jimmie before, it's hard to imagine for me now, after the feeling I had today with the very low grip and with a lot of problems with traction, how these cars will feel in an oval race," he said.
"Obviously there are no more traction demands, you are just on the oval and keeping the average speed. So that's a very different way to drive the car.
"Maybe the slide is less [dramatic] but for longer in the corner, and maybe that's a different feeling. The stop/start circuit like Bahrain, I think really hurt the philosophy of the NASCAR car."
Asked if the NASCAR experience had left him eager to test again, Alonso said: "For now I think it's OK.
"These cars are made for oval racing, so they stow a lot of weight on the left of the car, even if they try to set up for a road course."
Alonso did not rule out making a one-off NASCAR appearance in the future, but said the Cup series' 36-race calendar and the techniques required to be quick on ovals meant he would not consider a full campaign.