Sebastian Vettel admitted he was disappointed with Red Bull's final days of 2013 Formula 1 testing, but said tires were such a big factor it was hard to know how to feel about the car itself.
High tire degradation has been an issue for teams throughout the winter, with several drivers suggesting that the Pirellis' drop-off rate made it impossible to get the consistency needed for accurate car evaluation.
"We just have to look after ourselves," said Vettel. "We didn't tick all the boxes that we would have liked to and the last two days were not as good as we would have expected. But there's still a little bit of time left before the season starts."
Asked if he could specify Red Bull's problems, Vettel replied: "It's extremely difficult to pinpoint the exact areas because the tires were not consistent enough. If you sum up the whole three tests, we were all limited to what the tires could do.
"It was extremely difficult to read some setup changes and find a good direction with the car because the tires were simply not good enough."
Mercedes emerged fastest on the last two days at Barcelona, but Vettel said the situation was too nebulous to pay much regard to the times.
"We saw the lap time but it's a bit unclear at this stage which tires give you what kind of step," he said.