Button says he is reasonably happy with his new McLaren car
By Sarah Holt
BBC Sport at the Circuit de Catalunya
Jenson Button is confident his McLaren team can improve their car before the start of the season next month.
The world champion completed his pre-season testing programme in Barcelona on Saturday reiterating his belief that rivals Ferrari are the team to beat.
Button said: "The car is good but there is still room for improvement.
"I'm reasonably happy with it. There's more to come. There are areas we know we were a little bit weak in but they are areas we can change with set-up."
Button, who hands the McLaren over to team-mate Lewis Hamilton for the final day of pre-season testing on Sunday, tried out a major aerodynamic upgrade on Saturday, describing the update as a "good update - but we need more miles".
The 30-year-old's car was fitted with a new front wing, diffuser, engine cover and exhausts.
"I'm happy with the car that we have but I don't think we're getting the maximum out of the package we have," he added.
Button completed a race simulation at the Circuit de Catalunya on Saturday at the same time as Ferrari driver Felipe Massa.
The Englishman said after testing had stopped for the day: "If you look at the times today - the long runs - the Ferrari is competitive - but we all knew that.
"Ferrari will be quick. Fernando (Alonso) will definitely be quick as will Felipe (Massa). They are both very experienced drivers and very talented.
I think the Sauber will surprise a few people. I think it's quicker than most people think. I think they'll be up there at the first race
"They will get the best out of the car and we will do the same; we just haven't had enough time to get the best out of the package we used today."
Although everyone in F1 agrees about Ferrari's status - including
Mercedes driver Michael Schumacher
- it is believed that the top four teams are reasonably closely matched.
Button said making predictions about the order of the teams at the first race in Bahrain on 14 March was difficult because of the difficulty of reading the times in testing.
"I think the Sauber will surprise a few people," Button said. "I think it's quicker than most people think. I think they'll be up there at the first race.
"There could be five or six teams who could be competitive but it's all a bit of a guessing game at the moment and it will be until we qualify on Saturday in Bahrain.
"It's exciting and it's good for the sport and exciting for the fans and for all of us involved."