Kimi Raikkonen admitted he was not convinced by the wet-weather prototype tyres provided by Pirelli during the first day of testing at Paul Ricard.
The Ferrari driver completed nearly 100 laps on Monday, as Pirelli tried prototype rubber and last year's tyres side-by-side on an artificially-watered circuit.
The Finn said it was still early days to judge the performance of the new tyres, but conceded he felt more comfortable with last year's rubber.
"I think it's too early to say," Raikkonen told reporters after the session.
"We had the same as last year and some of the new ones, but I think we prefer last year's ones, but it's a bit too early to say."
He added: "I think it was okay. We haven't driven for a while. We can learn from this. It helps to understand more.
"I think here is as good a wet tyre test as you can get because it's similar conditions all the time. We help Pirelli and it's useful for us, so I think it was okay."
Sebastian Vettel will take over at the wheel of the Ferrari for the second and final day of running tomorrow.
Interview by Benjamin Vinel