Load capacity index 2,5 DTi Caravan at 18″

Does anyone know how high the TRAgability index for 18 inch rims for Opel Omega B Caravan 2.5 DTi is???? Is because different, with 195 and 205 at 15 inch are only 91 necessary, although at 17 ” already 94 is required! At 18 inch I did not find it!