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Carlos Sainz Jr has called on Toro Rosso to finally understand why his car fails to perform at the start of a Formula 1 race, when laden with a full tank of fuel.



The STR10 has recently looked strong through practice, to the extent that Sainz's team-mate Max Verstappen suggested only Mercedes showed stronger long-run pace during Silverstone's Friday running.

For the most part that form has continued into qualifying, but come the early stages of a grand prix the package has been found wanting.



Sainz has confirmed the car's handling characteristics are very different at the start with 100kg of fuel on board compared to when it is even at least 20kg lighter.



"Our first [race] stint is still our weakness. This is what we need to look into and improve," said Sainz.



"I don't know why. It's a shame because as soon as you're done with high fuel, you put on another tyre and you have 70 to 80 kilograms of fuel, suddenly the car starts behaving normally and we have very decent pace.



"We tested some things during the in-season tests in Barcelona and in Austria, but we still don't know exactly why this is.



"The only bit we struggle with is the first stint. FP1, FP2, FP3 and qualifying, we are always strong.



"But with tyres that haven't been warmed up correctly because the warm-up lap isn't ideal, our car looks to be out of the window and we don't know how to get it back."



Sainz has confirmed it is a personal feeling with the car, adding: "It's about how comfortable I am with the car.



"I know in qualifying I can do whatever I want, turning the car wherever I want because I have the rear support, I have the confidence.



"But with 100 kilograms I don't know if I'm going to oversteer on entry, understeer on mid.

"It's a bit strange."