Tires: P vs LT tires, P type tires, LT type tires

Question
Hi,
Your answer on P vs LT tires was very informative. I'm still a little confused. You mentioned to derate P tires 10% when used on light trucks. The tire doesn't know what vehicle it's used on. I expected the relationship of air pressure to load would be the same. I get the softer sidewall of P tire, thus lower pressures. The real question would be, if I replace a LT tire with max load (say 2500)@44 with a P tire with max load (say 2500)@35, am I ok to haul the same load. I am looking for soft ride unloaded without sacrificing load capacity or loaded controllability.

Hope that makes sense
thanks
Al

Answer
Al,

Let me explain it this way.

The 10% derating for P type tires has been in effect since 1968 - long before our current system of tire sizing was developed.  Everything since that time has been predicated on that - including the issues involved in the infamous Ford/Firestone situation a few years back.

Since it has been over 40 years since this was put in place, I can't ask those folks who did this any questions - I can only guess.  My best guess is that compared to passenger cars, trucks are expected to carry more load more often, plus they are more stiffly sprung.  Both of those things negatively impact the durability of a tire - hence the derating to accommodate common usage of such tires.

Continuing, perhaps I can best explain this with an example.

A P245/75R16 has a maximum load carrying capacity of 2271 # at 35 psi. for a Standard Load tire.  When used on a pickup truck, the load carrying capacity drops to 2065# (at 35 psi)

An LT245/75R16 Load Range 2205# at 50 psi.  At 35 psi, this same tire has a load carrying capacity of 1700#.  

In order to carry the same load a P245/75R16 can carry at 35 psi, an LT245/75R16 has to use 46 psi.

But the problem you've asked about concerns heavy duty trucks - and they are quite stiffly spring, but LT tires are sized with this in mind.  If you were to put a P type tire in its place, not only would the tire experience more of the road input compared to the suspension, but the extra flexing would cause some sway problems.

So I do not not recommend replacing LT type tires with P type tires for both durability concerns as well as handling issues.