Tyre pressures vary between bikes and the pressure range will be written on the sidewall of your tyre. The pressures will show a range i.e. 30-45psi or 80-120psi For lighter riders, to get a more comfortable ride or have more tyre compliance on rougher terrain (i.e over tree roots or rocks) you can run in the lower pressure range, however your bike will often be slightly harder to pedal and have a higher risk of punctures. For heavier riders, to roll faster or have better puncture protection you can run in the higher ranger however the ride will not be as comfortable and you may not have as much tread compliance on rough terrain i.e over tree roots or rocks. As a very rough idea often bikes will have the following pressures ranges: Mountain Bikes with tubes 30-45psi Mountain Bikes tubeless 25-35psi Kids Bikes 35-45psi Road Bikes 80-120psi Touring & City Bikes 45-65psi