Williams technical director Mike Coughlan admits the "very aggressive nature" of the Jerez track surface compromised the team's tyre data.
Williams opted not to run their 2013 car at the opening test, instead using last year's FW34 fitted with various development parts for this season.
The team cited learning about the new Pirelli rubber as one of the reasons for taking this route.
However, Coughlan concedes those results were "a little compromised" but the abrasive nature of the Jerez track's surface.
"We have had a very good four days in Jerez running the FW34," he said.
"We have gone through a complete programme testing various parts that will be used on the FW35. This compliments our current philosophy of using rigs to pass off systems before running them on the car.
"The drivers have enjoyed themselves running through tyre comparisons and set-up changes that you would be reluctant to do at a race meeting due to time constraints.
"Although these results have been a little compromised by the very aggressive nature of the track surface here, we have acquired some useful data to carry forward. We now look forward to the next test in Barcelona and the launch of the FW35."
