I've been having a little disagreement with someone that I hope you guys can help decide. He says that if you have a tire with a max load of 35 psi and you inflate it to 30 psi that it will be fine, which is all normal but the catch is he also says that if you took a tire of the same size but with a max load of 51 psi and inflated it to 30 psi, that it will be under inflated. His argument is that the tire should always be inflated to 80% of the max load or it is under inflated. The thing is that many people have tires with a max load of 51 psi and they are readily filled to around 30-35 psi (such as mine) and exhibit no wear symptoms of under-inflation. I may just not be able to understand it but I don't see how you can have two things of the same exact volume, inflate them to the same pressure, yet have one be normal and one be under-inflated. As far as I was concerned the max load rating was just how much the tire could hold before it started to become unsafe. Sorry for the length.
Let's start with some terminology. "Max load" and "Max load rating" refer to the vehicle's weight, not the inflation pressure of the tire. There is usually a maximum inflation pressure printed on the sidewall of every tire. The maximum inflation pressure is supposed to be a limit set by the tire manufacturer beyond which the tire may become dangerous to operate. They may also specify a pressure at which the tire achieves its maximum load rating (usually near the highest pressure). However, I have never heard anything suggesting that it bears a relationship to the recommended inflation pressures from the vehicle manufacturer. For years, tires have had maximum inflation pressures above 1.25 times the vehicle manufacturer's recommended pressures. If you look at the inflation placard, you usually see higher inflation pressures suggested for heavier vehicle loads. I think you are both partially correct because you're talking about two different things. The maximum load rating is usually achieved when the tire is near ('80%') its maximum inflation pressure. However, for normal use, (not a maximum load of passenger and luggage weight) a lower pressure is not unusual, nor is it underinflation.
Thanks, sorry about the mix-up on the terminology as well. I was just confused from what he was saying because I've always seen cars inflated to door placard pressure or a pressure that works for the driver regardless of the max inflation pressure and have never heard that being labeled as under inflation
