NASCAR Sprint Cup teams face an unpredictable race on Sunday at Michigan after briefly testing the tyres they will race on, following Goodyear's call to change the left-side specification due to blistering issues.
Many teams struggled to get the balance right during an additional practice session that NASCAR scheduled for Saturday afternoon in order to give competitors the chance to sample a harder left-side tyre.
An allocation of Goodyear's code D-4020, manufactured back in 2006, replaced the initial selection in the hope of stopping the blistering issues that arose from Thursday's testing session on the new surface of the track, which was subject to a US $7 Million repave project.
Goodyear officials reported that tyre temperatures on the alternate compound were down during the extra practice, also bringing speeds down due to the loss of grip coming from the tougher compound.
Mark Martin ran a 21-lap stint, more than half a fuel run, on the same set of tyres and Goodyear's Greg Stucker said there were no signs of the blisters that the veteran and others had experienced in practice.
Officials are confident of a trouble-free race as they attempt to avoid a repeat of the 2008 tyre debacle at Indianapolis, where a stop-and-go race occurred as tyres did not last a complete fuel run.
Former Michigan race-winner Greg Biffle, who had been fastest in practice with the original tyre selection, topped Saturday's evening session with a lap that was around 10mph slower than his best from Friday, and was the only one to break into the 195-mph barrier.
"The tyres are really hard, you can tell," said Biffle. "It is giving up a lot of grip. It is kind of funny, you can hear the tyres squealing when you first go out, before it builds temperature, you can feel it sliding on the race track.
"It slows the cars down and as it builds heat it actually starts to gain some of that grip back. I think the biggest thing with this tyre that I remember is the first three laps off the yellow as the hardest racing around other cars. The thing will just take off on you.
"That is the biggest thing we need to stay cautious of. After you get four or five laps in a run I think it will race pretty normal."
Teams were handed nearly half of their race allocation during the 75-minute session in order to to allow them to scrub some sets.