QuestionHi Barry,
I have several questions, but let me first give some background info.
3 years ago I bought a used 1999 Chevy Suburban (original owner Hydro Quebec) with LT235/75R15 Cooper winter tires on it. The tires were in great shape so to preserve them for winter use I bought a second set of tires (Hercules Terra Trac A/W LT235/75R15).
The vehicle’s tire placard states that the OEM tires were P235/75R15 XL with recommended tire pressure of front 32 psi and rear 41 psi. I always inflated the tires according to the specs on the tire placard. A while back, when I had the winter tires removed for summer, I asked my mechanic if they inflated the tires according to the tire placard specs. He said that 41 psi would end up at around 48 psi when driven and that the higher psi would wear out the tires unevenly. I think they inflated all the tires to 35 psi. Not being sure about his answer, I searched the internet until I found out someone who really seems to know what he is talking about.
On one of your posts (http://en.allexperts.com/q/Tires-2359/2009/4/Tire-pressure-load-correlation-1.ht...) you said “So in order to carry the same load as a P235/75R15, an LT235/75R15 has to be inflated to 40 psi front / 50 psi rear.” This was for a 1996 Ford F-150. (By the way the geocities link is now dead on that post; I did however find barrystiretech.com and I assume you are the same Barry.)
My first question is: what is the recommended psi for using the LT235/75R15 tires on my Suburban? The GWWR is 6800#; GAWR front is 3400# and GAWR rear is 3968#.
Second (and third) questions: are the LT235/75R15 tires better or stronger than the P235/75R15XL tires? In other words, what is the real difference between these tires?
The reason I ask is that the info sheet for Hercules Tire states that their LT235/75R15 max single load is 1985# at 50 psi, but their P235/75R15XL max single load is 2183# at 41 psi. I phoned Hercules 800 number and the person who took my call thought that this was a mistake since the LT235/75R15’s are 6 ply and the P235/75RXL’s are 4 ply thus implying that the LT’s should accommodate a heavier load. He said that I should probably inflate the LT’s to around 40 psi.
As you can see the info I have received has been less than scientific and empirical. I would appreciate your expertise on this matter.
Thank you very much,
Michel MacDonald
AnswerMichel,
Let's do the math thing first:
A P235/75R15 has a load carrying capacity at 32 / 41 psi of 1940# / 2183#. But since this application is an SUV, the load carrying capacity has to be reduced 10% to 1764# / 1985#.
In order to carrying the same load, an LT235/75R15 needs to use 44 / 50 psi.
Why? LT tires are designed to have high load carrying capacities compared to the space they occupy. Another way to say that is: High unit loading.
That means that the rubber has to be stronger - and to do that, the rubber is made less flexible. The net result is that LT tires do not like flexing, so to compensate, more pressure is used.
OK, so let's talk about this "ply" thing.
In the old days, the number of plies in a tire was a very important indicator of the strength of a tire.
But with today's modern fabrics, the number of plies has very little relationship with strength. In fact, you will find many tires with one or 2 sidewall plies, and 2 steel belts - and that's pretty much the standard construction for ALL LT tires. (A side note: Over the road truck tires - 18 wheeler variety - have a single steel ply - plus 3 or 4 steel belts), and yet they are as high as Load Range H (16 PR).
While earlier it was common to refer to the "Ply Rating" - a way of expressing the equivalent to the old way of number of plies - this has been replaced to remove the reference (and the confusion). P metric tires come in "Standard Load" (SL) and "Extra Load" (XL), while LT metric tires comes in "Load Ranges" - indicated by letters. If you look on Tire Rack, you will not find the use of the term "ply" or "Ply Rating" (PR). However you will find a "Load Index" which is another way of expressing the load carrying capacity.
This is a bit complex, but anyone using the term "ply" is just confusing the matter more - and just not staying up to date.
