Toro Rosso technical director James Key says the STR10 running this week at Barcelona is "almost completely brand new" compared to the Formula 1 machine tested so far in pre-season.
The car features a new Williams-style front nose for the final pre-season F1 test of 2015, and also new aerodynamic packaging, cooling and suspension.
Key told AUTOSPORT this version of the STR10 is the one the Faenza-based team would race in Melbourne in two weeks' time, as it chases a target of finishing fifth in the constructors' championship.
"The car itself is radically different here, so we have to start again [in terms of set-up]," he told AUTOSPORT.
"It's almost completely brand new: Aero, cooling, suspension and the nose is new.
"It's a Williams direction. We always knew this was the way to go - the only reason we did the long one was because this [new nose] is technically more difficult with the impact test, and we wanted to give ourselves a banker.
"We got the long one neutral in terms of aero performance, but that's all it was. It basically means you can run without having the stress of squeezing through a difficult impact test, but this one has passed pretty well.
"This and the rest of the car is actually the 'real' racecar. Everything before that was [built] months ago in terms of aero."
With Toro Rosso having two rookie drivers for 2015 in Max Verstappen and Carlos Sainz Jr, Key said the team had taken a deliberately conservative route to let them gather mileage initially.
"We just wanted a safe car [to get the drivers up to speed]. It was a conservative car to give us a baseline," he explained.
"This is the racecar and I'd have loved to have launched this one actually, because it's a big step forward compared to the launch car. This is what we've been working on."