The Austrian is the boss at Toro Rosso, the junior Red Bull team. Next year, the senior outfit Red Bull Racing will also be powered by works Honda engines.
"Our collaboration with Honda is fantastic," Tost told Marca.
"Honda is a very good and very serious company and they have improved a lot compared to last year."
Tost thinks it is realistic that Red Bull will win with Honda as early as 2019.
"To win races with our structure is difficult. Red Bull has three times the staff that we do. Everything is at another level, but Honda is a very successful company that wants to win races," he said.
"In Canada we had an improvement that gave us three tenths. We will have another in America and in 2019 there will be more. I am convinced that the power unit will be very competitive."
Many have scoffed at Red Bull's decision to become Honda powered, after the former top team McLaren failed over three years to make the Japanese collaboration work.
However, Tost said: "Luckily, McLaren was wrong. I already knew it last year when I said 'Jesus, how can you criticise such a fantastic collaborator and the money they put in?'
"We know that their chassis was not the best, so now when they were smiling at us, I think now it is my turn to smile."
As for Toro Rosso's drivers, there is a big question mark above Brendon Hartley's seat at present but Tost said that he is "very impressed" with Pierre Gasly.