A 120 pound person riding 29×3.0 tires on i50 rims on smooth trails might run 10 psi. A 220 pound person riding 27×2.2 tires on i20 rims on rough rocky trail might run 45 psi. Rider and bike weight, tire width, rim width, casing strength and thickness, trail conditions, and personal riding style all affect tire pressure. If you can tell us your weight, tire size and rim width, we can make a good guess. For example, I weigh 165 pounds, use 29×2.6 Trail tires on i35 rims, and run 17-18 psi.