You have a 60 series tire as the original size tire. I would not recommend you go to any 70 series tire for sure. You could go to a 65 series but you will need to install a 215/65-17 in order the keep the overall diameter as close as possible to the OEM tire. There is only .35% difference in overall diameter between the 235/60-17 and the 215/65-17 which makes it an excellent swap.