Mercedes driver Nico Rosberg says that cold conditions at the Circuit de Catalunya have exacerbated the tyre wear.
Rosberg was fourth quickest on the third day of running, completing 108 laps.
"We didn't have a perfect morning and it was tough out there with the low temperatures," he said.
"They were causing a lot of tyre graining, which made it difficult to do much useful testing. It was a few degrees warmer after the break and that gave a completely different picture.
"In general, I'm pleased with what we've achieved this week - I did a lot of mileage and the team has done a good job over the winter. I feel more comfortable in the car and the balance is quite good, but we still need to work in a number of areas. That's why I'm looking forward to testing next week and to see what progress we can make."
Lewis Hamilton resumes driving duties on Friday.
