George Russell and Lando Norris have issued a warning to F1 over hopes of improved racing action this year after failing a "perfect demonstration" during testing.
After spending much of the first two days of the first pre-season outing in Barcelona focusing on their own cars and getting up to speed with new regulations, drivers and teams have been able to try out following rivals on day three.
The new regulations are aimed at allowing cars to travel closer to each other through high-speed turns, in theory creating more overtaking opportunities and an improved racing spectacle.
Asked if the regulations have shown signs of working during his running, Mercedes driver Russell said: "I think the following has been improved but the slipstream effect has been reduced quite substantially so I don't really know.
"You obviously need that delta in the straights to overtake because you can only overtake at the end of a straight and into a corner.
"We can follow closer but from what we have seen, the slipstream effect is definitely less effective. We will have to wait and see.
"I got right up behind Lando, I was a car-length or two behind him and didn't catch the straight so that was slightly concerning so let's see."
McLaren driver Norris added: "Me and George had a perfect demonstration of it.
"I didn't want to let him past while he was on a quick lap so I screwed him over and held him up a bit. That is what you do in testing.
"George said it correctly. The following in the corner is an improvement.
"How much exactly is hard to know because you are always on different fuel levels and tyres and maybe when there are three, four, five, six cars ahead then it could be quite different again.
"But with all this obviously comes the deficit of a slipstream effect and how much that is going to be helping us.
"We can follow closer but then the slipstream is worse so a positive and a negative from it. We don't know which is the bigger effect at the minute so we will have to see until the first race."