Toro Rosso believe Honda's engine is close in performance to former supplier Renault's and the Japanese firm could wipe out the gap as early as halfway through the 2018 season.

The engine swap between McLaren and Toro Rosso for this year is thought likely to propel the 20-time world champions up the grid from ninth in the championship given the Renault unit is regarded as the third-best in the sport behind Mercedes and Ferrari.

But James Key, Toro Rosso's technical director, says he has been pleasantly surprised by what the team have found from Honda during pre-season testing.

F1 testing: The fastest laps so far

And Key suspects the power output between Honda and Renault is closer than has previously been reported.

"I don't know what Renault have done this year but we had a good feel for where the Renault was last year and at least from the numbers you see, which you always have to treat carefully because they are from different dynos etcetera, it's not so far off," he told Sky Sports News.

"Not nearly as far as was being suggested last year actually. It was probably pretty close to Renault towards the end of the season. And obviously they've made steps for this year and that's encouraging."

Honda endured a torrid three-year return to McLaren amid a succession of performance and reliability problems, leading the Woking team to call time on the lucrative works partnership.

How to follow testing on Sky

Key is optimistic the Japanese firm have built on their end-of-season progress last year and can wipe out the remaining deficit to Renault.

"The good thing about what we saw from Honda last year is despite all the difficulties they had over the first half of the season they still had the capacity to go and develop the power unit and it definitely was better by the end of the year," the Englishman added.

"That trend appears to have continued into this year and, as long as that trend continues, there's no reason why they can't genuinely be challenging Renault by mid-season or the end of season. That's what we'd like."

Toro Rosso giving Honda 'freedom'

While Honda have swapped one of F1's biggest teams for one of its smallest, Key believes the approach Toro Rosso have decided to take with their new Japanese colleagues is paying off already.

"We very much wanted to work with them," he said in a separate media session. "Our team principal [Franz Tost] knows them well and I've worked with them in the past so we felt we could have a very amicable relationship with them and a good understanding. So far that's very much been the case.

"[We are] trying to give Honda the freedom they need. Fundamentally we wanted to give them the opportunity they needed to get on with the job. I think they appreciated that."

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