How are the car bonus percentages currently calculated for each track?

You start with the number from last month, every car that didn't end up in the top 3 gets +1. The total amount of -'s needed to balance out those +1's (-15 if one car took all of the podium, -13 when there were three different cars) is split between the top 3 in such a way that the first place winner gets 50% of the -'s, second place gets 30% and third place gets 20%, so if one car won the lot it's -15, if one car got first and another got both second and third they both get -7. Then on top of that the track designer can add up to another +10% and -10%, freely distributable as long as there are the same number of +'s ad there are -'s, and no half percents.

We actually had a bit of a discussion about that a while ago. Since we changed from +3% for every month not won to just +1% the +/-10% from the track designer have the potential to wreck a car's chances for an entire season. So we were thinking of making them temporary. That is to say, the track designer still gives the bonuses, but at the end of the month they revert rather than carrying over. So in month A, which the Jaguar won, the Indy sits at -25% while the Carrera has 23%. The track designer gives the Indy the full bonus making it -14% in month B while the Carrera gets the full penalty for a total of 14%. Then the Indy wins the race. In the current system that would give the Indy -29% in month C and the Carrera 15% (the track designer for month C ignores these two cars altogether), while if we make the bonuses temporary the Indy goes all the way back to -39 while the Carrera gets 25%. So their bonuses are exactly as if no extra points were given or taken during month B altogether.

I've had some time to think about it, I figure this is the way to go. Especially since track designers tend to give their bonuses to cars that look like they could become relevant. This looks like a good track for an IMSA car but the Jaguar is all the way down at -15? Better give the bonus to the P962. The penalty meanwhile might be distributed among cars that are not going to be relevant this month anyway. So the system overall mildly favors whichever cars were already sitting in the competitive range.

We could even think about making the system less complex if it's temporary anyway, like maybe you don't have to give the same number of +'s and -'s, as long as neither is bigger than +/-10. Or there's a big, a medium and a small boost of +6, +4 and +2% and we leave the negatives out altogether, or...

I'm not particularly sure of the details yet, but I'm in favor of temporary track designer bonuses. In exceptional cases, like if we find out a new car was brought in at a way too low coefficient, we can have a special administrative bonus bump it up, rather than relying on track designers working it into their bonuses.