235/40-18
245/40-18
265/35-18
these sizes will retain correct speedo readings,but the 265 will be pretty tight,i had 255/40/17s and had no problems with 20 mm offset wheels altho the m5 with 18,s had 22mm offset on the rear(they were 9 or 9.5s).If you are trying to fill the space in the guards you may run into trouble but i guess its the same as lowering it with lower profile tires