Pastor Maldonado is confident Williams won't be penalised for opting not to run their new car at Jerez.
Out of the 11 Formula One teams, Williams is the only one not testing their 2013 challenger at the opening pre-season test.
The Grove-based team have instead been using last year's FW34 fitting with development parts for this year's car.
Maldonado, though, is adamant his team has not made an error in judgement.
"We are confident with the new car," Maldonado told Autosport. "But we want to be even more confident.
"With the test we planned to do here, there is no better way to do it than with the other car because we have a lot of confidence in it, a lot of data to compare and we are going to be more clear [in understanding the tyres] with it.
"We are trying small parts in the car, different things and different settings to try and understand the components.
"It is very important to be here with the old car."