Michael Schumacher believes that the four expected front-running teams are leading the way after this week's testing at Jerez, but admits that even they are still speculating about which is in the strongest position.
The seven-time World Champion drove on Thursday and Saturday, completing a total of 207 laps and finishing with the eighth quickest time of the week. Teammate Nico Rosberg was fastest on Wednesday but 12th on the overall list.
Schumacher says that Ferrari, McLaren, Red Bull and his Mercedes team appear to be the most competitive, as expected, but admits that it is difficult to make judgments from testing times.
"It's very tough for all of us to understand what is the order of teams," he said. "We have four that I think are the major teams, and others that look very interesting such as Sauber, the car is quite quick, and the Renault – there is a question mark how quick it is.
"If you look at other teams [on long runs] you have a certain drop off, that is tire-related. The good teams sort of stay all in the same window.
"But we are not able to be precise because you can play so much with the fuel load, and even with the ballast if you want to. So, there's a lot of speculation going on, even for us, but it's very interesting."
Schumacher added that he was pleased with the work Mercedes carried out on Saturday, with more dry running than in previous days.
"We had a good day," he added. "The weather handicapped us a bit in the early stages but we used it very sufficiently after that. We got the program through that we wanted to do – we understood the car quite a step more, so it was a good day for us."