Sauber's technical director James Key says his team has been "honest" about its pace during winter testing, not running with low fuel loads until the very end.
The Swiss squad showed strong form during testing, in similar style to last year, where it was also fast during testing, but then failed to repeat the feat when the racing began.
That has led some to believe the team has been running with small amounts of fuel during testing, but Key insists their pace is genuine.
He admitted, however, that it was hard to tell where the team will be when it goes racing.
"Performance is very difficult to judge because performance ultimately is relative to the competition," said Key.
"I think we have been quite honest with the way we have been testing, and ran lower fuel levels only on the last two days in Barcelona, which gave us a little bit more of an idea of where the car is.
"It looks quite tight but it is really difficult to judge. I think we are satisfied we have made the sort of progress we wanted to make over last year's car in some specific areas."
Key also said the team has significant upgrades planned for the first races of the season.
"The good news is the fairly big update we took to the second Barcelona test for the start of the season seems to have worked the way we hoped it would," he added.
"The important thing now is we continue to push and bring further updates as soon as we can. We have some big updates planned and will bring several new parts to the second race."
Subscribe to Autosport Plus
Register for free