I have the Kioti CK30hst with R4 tires which should be the same as yours. The max on the front 6 ply tires is 50psi and the max on the rear 8 ply tires is 30psi. Some Kiotis this size came with 6 ply rears. Being that I use the loader a lot, I keep the fronts between 45 to 50. The rears are normally at 20 unless I'll have my bh on for some time and then I bump them up to 25. My rears at 20 don't have full width contact but I worry about spinning a tire on the rim if I go to low.