World champion Sebastian Vettel is not sure Red Bull can catch up with Mercedes by 2015.
Red Bull boss Dr Helmut Marko said at Silverstone last weekend that, "In 2015, we want to be on par with Mercedes."
After taking part in the post-British grand prix test on Wednesday, German Vettel said: "The gap to Mercedes is very big.
"Until the beginning of next season we will try to catch up," he is quoted by Italy's La Gazzetta dello Sport, "and I hope that we are able to improve and get closer."
Vettel is also quoted as saying by Speed Week: "While we have made significant progress since the winter, the opponents also are not sleeping.
"We expect to...
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