Williams set to show hand in F1's final pre-season test

The team - which ended 2014 as Mercedes' closest rival on raw pace - has followed a low-key programme through the winter so far, focusing on reliability testing and procedures such as pitstops.

Valtteri Bottas said Williams would only ramp up performance work at this week's Barcelona test when its full 2015 package was in place.

"We have some updates to come which are going to change the car behaviour, so there was not really any point to go into the fine details of the car balance because once we start putting new bits on [at the last test] and for Melbourne, everything's going to change," he said.

"But we know there's nothing major wrong with the car that we can't fix by adjusting the set-up, so that's a good thing to know."

Williams's chief test engineer Rod Nelson said a qualifying simulation would definitely be part of the team's programme at the final test, though he hinted that his squad may still run more fuel than others for this.

"We'll do some quali sims, as most other people will," he said of plans for Thursday to Sunday's running.

"You'll set an hour aside some time and you'll go through a whole quali programme and at that stage we'll become clearer [to judge].

"But even then, people will have different fuel levels. Some of the smaller teams will have very little fuel indeed and some of the quicker ones will have a little bit more in hand."

Felipe Massa said that although Williams had not gone for maximum performance yet, all signs from the FW37 were good so far.

"I think we are on a good line," he said.

"It's always difficult to compare different fuel compared to other cars, so it's never easy, but I think the car shows good potential so I'm happy.

"The car is quite stable to drive. I think it's good.

"I still believe Mercedes is the best car. But we are there [in contention]. We just need to wait and see at the right time.

"The car is a little bit more stable on the rear [than 2014]. The traction is a little bit better, which helps some of the issues we had last year.

"We still need to improve - it's not at a perfect level, but it's a little bit better."