McLaren's Jenson Button and Lewis Hamilton have admitted they are struggling to be competitive in wet conditions at the German Grand Prix.
The team were uncompetitive in wet qualifying at Silverstone two weeks ago and the drivers said they were having problems in practice on Friday.
"Trying to get these wet tyres working is one of our big issues," said Button.
Hamilton added: "In the wet, we need to do a bit of an improvement on the wet tyres trying to get them switching on."
Button and Hamilton were eighth and 19th fastest in the second, wetter of two practice sessions at Hockenheim on Friday.
Button led Hamilton to a one-two in the dried first session, both men setting their fastest laps at different parts of the session, but when the track was dry.
The McLaren drivers said they were struggling to get the required operating temperature into both the 'extreme' wet-weather tyres and the 'intermediates', which are used in drier conditions.
Button said: "There's a lot of work needed to prove we can be quick in the wet because I think it could be wet tomorrow."
Hamilton added: "It feels like you're on ice when you're out there. Maybe the Ferraris and Mercedes are seeming to switch their inters on a bit better, so we have a bit of work to do there."
McLaren have brought a major aerodynamic upgrade to this race - with new sidepods, brake ducts and diffuser.
Button said the team were "happy that it's working" but Hamilton admitted they would not know how effective it had been until they got some extensive running in the dry.
"The car definitely should have made a slight step forward but we don't know how much - we might not know until tomorrow or the race if it's wet on Saturday," Hamilton said.
"I couldn't tell you how big a step it was but I'm quite confident it is a good step."
Sporting director Sam Michael said the team were confident of a strong weekend.
"We come here expecting to fight for pole position and win the grand prix," he said. "And I think we would have taken that approach even without these updates."