A funny thing happened at Thunderhill last March. It rained. This is unusual in California but that is not what made it interesting.
There were several FC cars on the pace lap. By time we were on the back strait on the pace lap it was pouring. Not having rain tires two of us pulled off. There was a a three or four car pile up after the green in turn one.This led to a red flag. They gave every body 15 minutes to switch to rains and then restarted the race. The car running in P2 spun into the weeds and stayed there for there rest of the race. The guy who was well into the lead was black flagged for sound on the last lap. There were 2 of us left at this point. So they were in impound P1 and P2. Or so we thought. Then it was pointed out that the rain tires on the remaining cars were illegal. They were too wide. The drivers of these cars were unaware that their tires were illegal but that did not matter. So in the end nobody got points for that race.
Moral of the story: Make sure when you get rain tires that they are the right size. Many of us just go to the tire vendor and ask for the rain tire for their FC. Caveat Emptor!!