Thanks RMi_wood.the_car_folder/extension/ext_config.ini
[SHADER_REPLACEMENT_...]
ACTIVE=1
MATERIALS=the_tire_material_name
PROP_... = ksSpecular, 0.01 ;<--- lower values has less shine
Finally, I solved the problem.
As it appeared only for the skins, I thought that could not be caused by a general file as ext_config.ini.
I finally renamed 2 files in each skin folder to deactivate them : vrc-tyres.dds and vrc-tyres-blur.dds and that's fine.
Another question:
I wanted to convert the kn5 of these cars to open them in blender and find the tyre material name, but I was not able to make the conversion.
Is there another solution to find the material names?