Red Bull's 2019 drivers do not expect Honda to have a field-beating engine at the beginning of next season.
Pierre Gasly, who has been using Honda power all year and for 2019 steps up to the senior team, said: "There is still a gap to Mercedes and Ferrari.
"Honda knows they have to make up ground. They are very open about that," said the Toro Rosso driver, according to Speed Week.
"But I know they are doing everything to be as well prepared as possible for 2019. They will make another significant step forward next season."
Frenchman Gasly said progress in F1 is not always linear.
"Sometimes you have only a few steps for months, and then you have a breakthrough and suddenly it is four tenths on the simulation," he said.
"I think Honda has already caught up most of the gap, but I do not expect them to be on par with the other engines at the start of the 2019 season. But they will reach that level.
"Honda is investing a lot and developing very intensively," Gasly insisted.
Gasly's 2019 teammate, Max Verstappen, agrees. He rejected paddock suggestions that Red Bull is grossly overstating the progress Honda has made in 2018 with Toro Rosso.
"What we are saying about Honda is not a lie," he said.
"We see all the numbers, but of course it also depends on the progress of our competitors. I am a realist," the Dutchman added.
"I estimate that we will lag behind in terms of performance at the beginning of the season, but we will definitely be better off than we are today," said Verstappen, who has been an outspoken critic of Red Bull's customer Renault engines.
"Whether it's enough to get involved at the top, we will have to see."
