Bianchi has made a strong start to his F1 career, finishing in 13th in the Malaysian Grand Prix.
But as he only had a day-and-a-half of testing after securing a last-ditch drive with the team after Luiz Razia hit funding problems, Bianchi suspects there is more to come.
"I don't want to say I will gain one second or anything like that, I'm being realistic," Bianchi told AUTOSPORT.
"In Melbourne, I was 90-95 per cent ready and already feeling really confident with the car.
"In Spain, we will be at 100 per cent, when everything will come together and we bring some new pieces to the car."
Bianchi added that it is only around a tenth of a second that he has still to find.
"My driving is already good but I need to work on it and maybe I will find one-tenth on my side, which would be good," he said.
"It's just confidence in the car as we didn't run much (pre-season)."
Bianchi is confident that Marussia does have the development potential to continue to close the gap to the midfield.
While staying ahead of Caterham is the team's priority, the gap to the back of the midfield is smaller than it has ever been.
"I'm optimistic because every time we bring something, it has always gone better," said Bianchi.
"Now we have to see when we will bring the new parts to the car and if it will give us the possibility to close the gap to the midfield.
"It's an encouraging start."