Firestone air spring kit 4101 has an operating pressure range of 10-60 psi. The optimal pressure setting will vary with how much weight your are carrying and the condition of your coil springs. If you are towing a trailer we recommend inflating the air springs to 15-20 psi, then connecting your trailer. You can then inflate your air springs until you get your desired height and ride that you are looking for. If you have an onboard air compressor system it is easier to accomplish. After a awhile you will know what is optimal for your setup from testing a few different psi settings in your air springs.