This year's 60 per cent tyre deform differently in the corners, which has a great influence on the aerodynamics.
Apr.21 (GMM) Pirelli's new 60 per cent scale wind tunnel tyres could be to blame for some teams' troubles in 2013, according to a technical expert.
Top team McLaren faltered badly over the winter in switching from the winning 2012 car to the new and uncompetitive MP4-28.
And it's been a similar story at Williams and Sauber, who last year were among the standouts of the midfield and are now conspicuously scrambling for pace in 2013.
"It has to do with the new Pirelli tyres. They are more angular than in 2012, and then deform differently in the corners, which has a great influence on the aerodynamics.
"Millimetres play a role, but the 60 per cent tyre for the wind tunnel models have led McLaren and Sauber astray," he added.
An unnamed McLaren engineer agreed: "They (the Pirelli tyre models) are much worse than last year."
Faced with the latest criticism, Pirelli's Paul Hembery insisted: "We are talking about eleven teams with eleven different wind tunnels.
"It is not easy to build good 60 per cent tyres," he added.
The theory would explain why teams who made significant changes over the winter have disproportionately suffered so far this season.
Meanwhile, Sauber boss Monisha Kaltenborn has defended struggling team rookie Esteban Gutierrez. On Saturday, the young Mexican failed even to make it out of 'Q1'.
"Clearly neither he nor we can be happy so far with the results that he's shown," she said.
"But we have been very convinced, and still are, of his talent, so it's for us now to make sure that he has the surrounding (environment) that he can develop."