The point is, even if the sidewall spec happens to be the amount of air you are using, it is not how you get there. The correct amount of air pressure in the tire is based upon the axle loading divided by the number of tires per axle, and correct size of the contact patch. You get there by either using the vehicle manufacturer's spec (i.e. door jamb tag), or by one of the several emperical methods for determining the air pressure, like the chalk method or the card trick.
The sidewall spec tells you what the maximum load allowed on that particular tire is, and it is determined solely by the tire's internal construction. Practically speaking, if you need to put in a higher pressure than the sidewall spec to get the needed contact patch size, then you are overloading that tire and need to either lighten the load or upgrade to a heavier duty tire. Conversely, if you are at the max pressure listed on the sidewall and are getting too much tread contact and sidewall bulge, then you also need to lighten up or install heavier rated tires.
Another point to remember when you upgrade from stock tires to heavier duty light truck rated tires like many of us do with our Jeeps (the stockers are "P" rated aka passenger car tires), that just because the tire can take the additional load, it probably isn't a good idea to exceed your rigs GVWR since other factors come into play like suspension, wheelbase, etc. The tires are just no longer the weak point. The main benefit of switching to LT rated tires is the increased durability, which actually is just a by-product of the additional internal structure needed to carry the additional rated load, even if you can't use that capacity for other reasons.
Think of it like installing heavy duty drivetrain parts, where you are just shifting the weak point around to different components until you reach a point where it doesn't break anymore.
---------------------------------------------------------- Del "Mighta been an engineer but for the Calculus" Rawlins del@_kills_spammers_rawlinsbrothers.org Remove _kills_spammers_ to reply via email. Unofficial Bearhawk FAQ website: