Lewis Hamilton thinks Mercedes could trail Ferrari by as much as half a second as Formula 1 pre-season testing draws to a close.
Ferrari boasts the quickest time from the two weeks of running at Barcelona so far, with Mercedes' best time - set by Valtteri Bottas on the final morning - currently half a second slower.
Hamilton believes Ferrari's advantage is real although he admitted he is "not quite sure" what the true gap is.
"I think it's potentially half a second," he said.
"Something like that, potentially.
"We will be analysing a lot from this test and there will be some modes that we try to implement.
"There's not a lot of time hopefully we'll gain a tenth at least just on our understanding of the car.
"The car is old now, it's worn and torn, so we'll have new components.
"All that stuff will come along. So, it's interesting."
Fuel loads and engine modes can contribute to dramatic swings in lap times, which makes it almost impossible to make direct comparisons in testing.
Hamilton said he was not particularly concerned by replicating Ferrari's lap times as "there's no reward for being quickest in testing so it doesn't really matter".
Mercedes introduced a new aerodynamic package for the second week of testing at Barcelona, which Bottas said on Thursday had improved its weak points.
Hamilton agreed with his team-mate but when asked by Autosport if he expected a half-second advantage to be real, or be reduced by the season-opener in Australia on March 17, Hamilton said: "I have no reason to expect that to come down.
"The testing, you have to take it with a pinch of salt. We could get there and it could be bigger. We could get there and it could be less. We could get there and it could be equal.
"I really have no idea. There is no way when you are looking at the GPS to know what fuel load they are on, or what engine mode they are on.
"They are faster on the straights than us. Is that because they have their flexi wings that they have had in the past that drops drag, or are they more turned up than us, or are they more efficient and have less drag than us in general, or are we heavier?
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"We have no idea we really won't know until we get to the race.
"Of course naturally we hope that it is definitely not bigger than what we might see now and we hope it is better.
"But we cannot bank on that so we have to work towards trying to close the gap that might be there, or is there."