The German said the team have a good gauge on how they compare to the likes of Ferrari - while several new designs and innovations have fuelled his own belief in how competitive the Silver Arrows will prove in the coming season.
“The car is quick, and there are some real innovations on it - I'm very impressed,” he told reporters in Barcelona. “You can really see the progress, how far we have come.
“There are so many things where we are ahead of the game. I am not saying we are the quickest, but there is some good stuff on the car.
“We’re paying attention [to rivals], of course. We have strategists who are doing all the calculations already, so we more or less know where we are - because they are able to see full tanks and so on, after a while they get a picture of where everybody is.
“So we know where we are, within a band of errors. Would you like to know? Sorry, I'm not going to say!”
Rosberg added that Mercedes’ focus on mileage rather than performance had been validated by the discovery of several small issues they may not have otherwise detected.
“We found a few bits and pieces that broke on the car, and we wouldn't have found them if we'd only done 2000kms.
"So that has been useful, because for sure that will help us now in the beginning of the season. No race-stoppers, but still things which you don't really want to see happen."
Mercedes completed 3,142km over the four days of running in Barcelona – considerably more than Toro Rosso, who logged the second highest tally with 2,080km.