Ferrari has brought a raft of Formula 1 upgrades to the United States Grand Prix, but only Sebastian Vettel is running the full package.
The Italian team has introduced a new diffuser along with revised front and rear wings and an updated floor at the Circuit of the Americas.
Autosport understands it only has two diffusers, one of which is on Vettel's car with the other kept as a spare for the four-time world champion.
Kimi Raikkonen is running the old specification of diffuser and also the older floor.
But the Finn does have the new front wing, which features endplate modifications aimed at creating more downforce.
The new floor is modified in the central section of the diffuser, making the diffuser work harder at a higher rear ride height, as well as the slot arrangement in front of the rear tyres.
Ferrari currently trails Mercedes by 145 points ahead of Sunday's race and must outscore its rival by 17 points to extend the constructors' fight to Mexico City.
The team has only managed to achieve that feat once this year, when it scored a one-two in Monaco.
Vettel will start the race second on the grid, behind title rival Lewis Hamilton, with Raikkonen starting fifth.
The other Mercedes of Valtteri Bottas will line up third.