The answer to this question depends on several factors, including the type of vehicle you are driving, the tires you are using, the terrain you'll be driving on, and your personal preferences.
If you're driving a light duty vehicle on pavement, you can probably get away with airing down to 20 PSI or even lower. If you're driving a heavier vehicle, such as an SUV or truck, you'll want to air down to around 35 PSI. And if you're driving on off-road terrain, you may want to air down to as low as 10 PSI.
Keep in mind that the lower you air down, the more comfortable your ride will be, but you'll also have less traction and may risk getting a flat tire. So, it's important to strike a balance between comfort and safety.
Ultimately, it's up to you to decide what PSI you air down to. Just make sure you do some research and consider all the factors before making your decision.