1 Lamborghini
1 Nissan
1 Ferrari
1 Porsche
1 Mini
1 more randomly selected from the five car types above (e.g. selected using a dice)
In checking our database, I've established the following fastest ever recorded laps for each class:

Cars will be placed on the grid in this order, however the sixth car will be placed in the grid position directly after that of the same type of car, with the other cars moving down one position if necessary.
As usual, the drivers will roll a dice to determine their first race starting car and grid position, thereafter moving down one position as is the norm for IROC races. The cars will start all races from their original grid positions. Also as usual, each driver gets to drive each car, however because of the varying performance differences, driving with have to focus on overtaking without driving through anyone ahead. The current penalty for driving through another car will apply - the length of the pit fence back from where the driven-through car stopped. Races can be 10-15 laps in length.
So, does this sound interesting to any of you?