Toro Rosso running updated 2015 car in first Formula 1 test

Toro Rosso secured a late deal to replace Renault with 2015-spec Ferrari power units and that put pressure on the team to get a new car ready on time.

It appears the Italian team has brought last year's car, modified to fit the new engine, with some new parts in the interim before the 2016 car is unveiled for the second test, which starts on Tuesday.

"We put a car together that was a mix of last year and some parts from this year," said Sainz. "At the moment, it's an STR10 with parts of STR11.

"I don't feel any performance yet but that's because it hasn't arrived.

"I will tell you a bit more about how it feels next week when I evaluate better all the new parts that are coming that will fit together to make a full STR11."

Sainz clocked up an impressive 161 laps, more than any other driver on day three of the first test in Barcelona.

Reliability was the Toro Rosso's Achilles heel last term so Sainz was pleased to have had a productive day.

"We are doing more mileage every day and this was the target when we came here," he said.

"We didn't look at anything other than reliability.

"That was our weakest point last year and at the moment I'm confident that the car is lapping and lapping and lapping and not giving big issues.

"We are having small issues, very, very small issues, that have to be addressed but apart from that there is progress and confidence that we could have better reliability this year.

"We were not looking at performance. I don't even know the laptime or what position I finished today.

"Hopefully we can give a bit more of a taste next week when we have all the parts together, we have the full STR11 together and we see how it handles."

Sainz pulled over at the exit of the pitlane during the session, bringing out the red flag, but the Spaniard said he did so as a precaution.

"I was practising pit stops and the team were not sure if one of the tyres was fully attached so they just made me stop at the end of the pitlane.

"I'm not referring to that as an issue. We had a lot of small issues internally but this particular issue is not my main concern, it was just a pitstop."