Nico Rosberg believes Mercedes has made good progress in understanding ties at this week's Mugello test, but thinks his team needs to do more to improve its high-speed corner performance.
With its experience in the first four races showing how important tire management is, Rosberg and Mercedes spent a lot of time on the final day working on improving its knowledge of that area.
Speaking at the conclusion of testing, Rosberg said he was upbeat about what could be expected for the next race in Spain - although he was well aware that other progress was needed.
"It has been a good testing program for us here in Mugello," explained Rosberg, who won the Chinese Grand Prix. "Today, we learned a lot about tire management, and this should help us at the next race in Barcelona.
"There is still a lot of work to do of course, and we do need to improve our car in fast corners, but overall I am pleased with the progress that we have made here this week."
Although there have been some suggestions that teams have not got as much out of this week's test as they could have hoped, Mercedes team principal Ross Brawn was happy with how things had gone.
"The first opportunity for in-season testing for some time has proved beneficial, particularly as it has enabled us to test some new development ideas and take a closer look at improving the car from things that we have identified over the first four races," he said.