Valtteri Bottas says he is confident Williams has made a bigger step forward with its new FW38 than it did with last year's FW37 despite a low key showing in pre-season testing.

The Finn - who finished fifth in the 2015 F1 standings - drove the car on the opening two days of the test, breaking in the FW38 before handing over to team-mate Felipe Massa on the final two days.

A low key test for Williams as it focused on reliability and understanding the car, though the FW38 has been described as an evolution of last year's car, Bottas says the changes are substantial and represent a bigger step forward than it may have seemed.

"First impressions are good," he told Crash.net. "The car feels like we have attacked well the areas we were weak and the engine feels better than last year but that is the only feeling so far. We haven't had any facts of where we are in relation to others. I am sure we can improve the car a lot. I feel a bigger step in the improvement than what we did last year."

Key to development of the FW38 has been an improvement in low and medium speed mechanical grip after the team struggled at certain circuits in 2015. As it stands, Bottas says the early impressions of the improvements are positive.

"It was pretty obvious mainly in slow speed and medium speed corners we were missing cornering performance so I think we have improved that. If it is enough I don't know but it feels better."

"It is an evolution but there are some interesting work items to do next week on the mechanical side, which is a different approach to what we had before. We have a lot of improvement planned for the beginning of the year. We are just waiting to see where we are and how fast we are."