The aerodynamic developments IndyCar will allow teams in 2019 will not be defined until August at the earliest.
IndyCar is set to open up development 'boxes' for teams but it has yet to finalise what can and cannot be altered from the spec, a process requiring team feedback.
Series president of competition and operations Jay Frye told Autosport that the areas of potential development will be defined "in August at the earliest - maybe later than that".
"We have talked about which 'boxes' we're opening on the car for the teams to develop within," said Frye.
"And about three-quarters of the way through the season we'll talk to the teams like we always do and ask what adjustability they don't have in the car at the moment that they would like, going forward, to get more potential out of it."
Frye and Bill Pappas, VP of competition and race engineering, confirmed IndyCar will also listen to input from teams on what they would like to see incorporated as part of the spec Dallara package.
"Yes, there are changes you can make across the board," said Frye.
"If you look at the rear wing on the superspeedway car, for example, it's smaller than in the original drawings of it because when we did the first tests we discovered it needed to be reduced to reduce drag.
"So there's things that we can do as we go, if a piece isn't working as we intended."
Pappas told Autosport: "First of all, we have to respect what Dallara is doing for us as series partners. But we'll take information and input from the teams.
"But as yet we haven't heard yet from the teams what changes they would like Dallara to make spec for next year.
"Some of them want to go their own way in certain areas, some are happy with the way this package has already played out and the level of [customisation] they can already make - for instance, with shock and damper programs.
"Personally, I think what areas should be spec and what areas could be opened for next year is something that can't be fully defined until we see how this car, this aerokit, reacts on each track we go to.
"So that's why it could be after the season that the final decision is made."
Pappas also confirmed IndyCar will return to the rear wing variations seen at Indy in May, with the underfloor strakes back in place, for August's Pocono race.