Spanish Grand Prix polesitter Nico Rosberg says he is struggling to understand why the W04 performs so differently qualifying compared to the race.
Rosberg secured his second successive pole position on Saturday but tumbled to sixth in the race, finishing over a minute behind winner Fernando Alonso.
"I didn't expect it to be that tough," he said.
"I thought we would be in a better position. It was really difficult out there, but why? Why on one lap are we so fast and on a long run so slow? It's strange, and it's not just one axle, it's front and rear. Both are struggling and there's no explanation."
Rosberg added that he was stunned at the way he dropped down after leading the opening stint.
"After the first stint I thought 'who knows, maybe I can keep them behind all day? I knew they were quicker but I was quite comfortable and doing what I wanted to do"
"But then I just went backwards so badly. It was unbelievable. It's so strange, so inexplicable. I'm working with my set-up just towards the race, not even for qualifying, and we're so fast on one lap and then in the race so slow."