Car class;
The car converter is the simplest but a little explanation won't hurt I think. All you did is good but you miss one step. Changing the physic extension of a carfile is not enough, you also need to edit it (with WinMip2) and change the line "car_class": 
"car_class = 1" for cup 
"car_class = 2" for gns 
"car_class = 3" for cts 
"car_class = 4" for pta 
I hope you kept a back-up of your original cars, it might be a mess if you try the converter on cars with a "wrong" file extension.



================================= 
Quick reference for Physics Type 
Cup physics 
car_classes = 3 (in series.def file) 
type = 0 
car_class = 1 (in .car file) 
Physics extension = cup 

Busch Grand National Physics 
car_classes = 5 (in series.def file) 
type = 1 
car_class = 2 (in .car file) 
Physics extension = gns 

Craftsman Truck Physics 
car_classes = 9 (in series.def file) 
type = 2 
car_class = 3 (in .car file) 
Physics extension = cts 

Papyrus Trans Am Physics 
car_classes = 17 (in series.def file) 
type = 3 
car_class = 4 (in .car file) 
Physics extension = pta 

=====================================