McLaren's Jenson Button has said his former team-mate Lewis Hamilton's Mercedes team are in good shape for the new season.
However, Button admitted it was hard to interpret pre-season testing times to establish who had the fastest car.
"I really don't know who's quick," said Button. "But the one team that has been consistently quick has been Mercedes more than anyone else."
Hamilton says he has deliberately played down Mercedes' form.
McLaren still have "room for improvement" with their car, Button added.
"I like driving it," he said. "It is a nice car to drive. There is nothing that scares me with it but I still feel we can do a lot more with it than we are at the moment, and that's the exciting part, that we're not at the end.
"We're not ready to go racing yet. There are still interesting things that we have to try over the next couple of days."
He said the unusually cold weather conditions in testing in Spain over the last month and occasional rain had disrupted McLaren's preparations.
He added the team needed the final two days of the test this weekend to get on top of the car.
"If we have two straightforward days it is enough to be ready [for the first race] but I'm sure we can deal with it and still do a good job," he said.
"But I feel there is so much more we can achieve with this car. It's just been a strange old winter in terms of weather and things taking longer than we expected."
World champion Sebastian Vettel said he was pleased with his Red Bull.
"Generally we are happy with the balance of the car," he said. "I think it is a better starting point than we had last year.
"It is very difficult to know in terms of pace where are.
"We have a rough idea, it's fine, we are happy with the balance and the way the car behaves but whether that is enough we don't know at this stage. It will be close until the end and we are not even at the beginning."
Ferrari's Fernando Alonso, who narrowly missed out on the title last year, said he was optimistic he could have a strong season and fight for the title.
"I don't see any reasons to not think so," Alonso said, "because last year it was a very difficult winter, we were completely lost and with that car we fought for the world championship all the way to Brazil.
"We now have a car that is responding well to what we change, a car that is doing what we expect the car to do.
"We know we are maybe not the quickest still but the starting point or the potential of this car is there.
"Last year it was a dream to make that car work. This year to make this car work I think will be much easier."