I have Kyosho StadiumForce TR-15 and stock tires wore out so I had to change them. I got cheap some tires and they are ok except they are little bigger in diameter than the old ones so car is a little sluggish to get to speed. I calculated that by changing the clutch bell from 13T to 12T I can compensate enough to get the gear ratio back to original.
The problem is that Kyosho doesn't make 12T clutch bells and I don't know what other clutch bells fit to that motor. I hope someone here can help, because I've been asking around and nobody been able to help or wanted to help.
If possible, maybe you could point where to get that clutch bell.. Maybe Tower Hobbies or other hobby shop.