McLaren's Jenson Button unsure of 2015 Formula 1 pecking order

Fastest times on the eight test days so far have been shared between Ferrari, Sauber, Lotus and Red Bull, although reigning constructors' champions Mercedes have set ominous times on harder tyres and heavier fuel.

Button said that even by normal standards, 2015 is difficult to predict.

"I haven't got a clue what is going on in terms of who is quick and who isn't. It's really difficult to read," he said.

"You can expect Mercedes to be quickest looking at the long runs. I think Ferrari look pretty strong, but then I don't know where Williams is.

"Sauber looks reasonably good, which is good for them. Red Bull, yeah they look pretty good.

"Last year, you could read it, you could see Mercedes and the Williams were second quickest and us third. Whereas this year, I haven't got a clue."

He added that different cars' behaviour on different tyres was also blurring the picture.

"It's tricky. Put the soft tyre on and for some they gain one and a half seconds, for others it's a few tenths," Button said.

"It's random at the moment. I think it makes it great for the first race."

BOTTAS: FIELD HAS CLOSED

Williams driver Valtteri Bottas reckons that the field has definitely closed up for 2015.

"I think we need to be aware of everyone really at this point," he said.

"It is clear that Ferrari has improved, also possibly Red Bull has improved.

"We know we have improved from last year and we know Mercedes is going to be really strong again ­that's for sure.

"I just have the feeling everything's going to be closer this year.

"In terms of where we're going to be, that's still a question mark. But I think we're going to be there or thereabouts."