Hey, Fabio.......
I've had that problem with some Tamiya kits as well. Seems that the tires crack where the molten rubber flows through the mold and meets itself on the other side opposite the sprue. Maybe the rubber gets too cold when it meets. What I do is make an RTV mold of the tire and cast resin copies. I've had fantastic results so far.
If you want, you can send me one of the tires and one of the rims and I'll make a set for you, or I can give you the knowledge needed to do it for yourself.
email me at leemitcheltree@bigpond.com and we'll discuss it further.