Pascal Wehrlein has said that he thinks predictions of a five second per lap boost for Formula 1 next year could be right.
The German is among only a handful of drivers who have been taking part in Pirelli's testing of the bigger and wider slicks next year.
Wehrlein's latest running took place in less than ideal weather in Barcelona, but he said afterwards: "I can't say much, but yes - the tyres are faster."
Although racing for Manor, Wehrlein remains closely involved with Mercedes, who along with Ferrari and Red Bull have modified 2015-spec cars to assist with Pirelli's 2017 testing.
Along with the bigger tyres, F1 cars are also getting substantially more downforce for next year, leading Wehrlein to predict: "Five seconds per lap [faster] is quite realistic.
"Whether it is six seconds or four, we will have to see, but it will be much faster. On top of that, the wide tyres look much better."
Some figures, including Pirelli's Paul Hembery and Sebastian Vettel, have said that the 2017 improvement is hard to quantify, particularly because the 2015 test cars are not yet producing enough downforce.