It’s this extra surface area touching the ground that makes all the difference. People think that the sideways growth from dropping their tire pressures makes the difference, but in reality it is the length that makes all the difference. If you calculate how much your surface area grows in length, you will be astounded. Would you also believe it if I told you tire deflation decreases the chance of a puncture in your rubber?
To prove it, take a balloon and blow it up until it is tight. Then, take it to a sharp rock and carefully push down on the balloon. You will find that it pops very quickly. Now, take an identical balloon and blow it up half way, and do the same thing with the rock. You will find (amazingly) that the balloon will not pop. By letting air out of your tires, you are allowing the tire to mould itself around the rock and thus decreasing the chance of a puncture. However, I must point out that by letting the tires down you are increasing the chance of sidewall damage if you allow tires to scrub on rocks.
Tire Pressures need to be decided on based upon the weight of your vehicle, size of the tires and most importantly the terrain that you are driving on. For most vehicles, beach driving should be done at 15 – 18 PSI. For gravel driving, around 23 – 26 PSI is usually acceptable. Rock and mud driving usually requires similar pressures to sand driving (perhaps a little bit more). I’d suggest you test the difference of having your tires at 20 PSI then letting them down to 15 PSI. Sure, its only 5 PSI difference, but it is actually letting out ¼ of the air in the tire and this is what makes all the difference.