Tires: Tires Replacement, capacity curve, true capacity

Question
Hi again (and don't worry, this would be the last, jeje)

First thank you very much for be so diligent and patient, this have been a "little" long compare to the kind of posts you recive.

My last question: What is the load capacity of the JATMA listed 255/70 R16 tire @ 26 psi, and what is the equivalent pressure for the TRA's LT255/70 R16  tire.

Have a good night and a happy weekend

Thanks.

Best Regards.

Daniel Figueira S.
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Followup To

Question -
Thanks Again, but I need to ask you some things that I still don't understand and others that I don't know

1. From this: "So an LT255/70R16 per TRA has an equivalent load capacity at 29 psi, 32 psi, and 35 psi.  (yes, I know that's 2 psi higher except for the last point, but that's what the table says!)" I understand that for this tire (LT255/70) at 29 psi the load capacity is 752 kg and at 32 psi (210 kPa) the load capacity is 810 kg.  And just so we are complete for 35 psi (240 kPa) it's 875 kg.

So I should inflate mines at 29/32 (front/rear) psi (+ 3-5 psi by your recommendations). If this is correct, I don't understand why if my tires say that they can support a 1000 Kg. (2205 lb.) load at 50 psi, they need so little pressure to carry 752Kg.(@29psi) and 810 Kg.(@32psi).

If all the above is correct and is only a matter of tire's load capacity curve, I want to know wich are the equivalent pressures in a P255/70 R16 since Goodyear sells the same Wrangler RT/S in this size, I mean they sells the P and the LT, and the P has a load capacity of 1030 Kg. (2270 lb.) at 35 psi (I know that the true capacity is the 91%=936 Kg=2065 lb.). I ask this because in my home we have the 5 doors Land Cruiser too, that came with the Wranglers RT/S P255/70 R16, and we replace them with the LT kind, and since the manual says that the size of the tires are 255/70 R16 (without P or LT) and they need a pressure of 26 psi front and rear, We don't know what pressure   would be the correct.

2. You always say that for P tires the minimum you recommend is 26 psi, so you can tell me wich is your recommendation on LT tires.

Thanks again, sorry for the extensive, but I care too much about car safety since ours road are in very poor conditions.

Best regards.

Daniel Figueira S.

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Followup To

Question -
Hi again, sorry for the poor information, but let me tell you the real problem:

I'm from Venezuela (yes the same country with the mad president that goes crazy at the ONU - Sorry for that I apologize for him), so you can tell the way that things work around here. I just bought a TOYOTA LAND CRUISER PRADO RZJ-90 3 doors (not sold in U.S.), and came with Goodyear Wrangler RT/S LT255/70 R16 tires on 16x7 wheels, but the manual (no placard on the vehicle) says that for my model the tires are 215/80 R16 (from my research they came on 16x6 wheels, and have been sold in australia) with 27psi front and 30 psi rear (35psi under load), I think for the pressures that is a P tire. FYI in the same manual the tires for the 5 doors model (VZJ-95) are 255/70 R16 with a pressure front and rear of 26 psi, so they must be P type.

So i want to know the equivalent pressure for this tires (LT255/70 MAX. 50 PSI) and for the P type too, since I wrote to TOYOTA Venezuela with no answer.

Thanks Again and sorry for my bad english.

Best Regards.

Daniel Figueira S.
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Followup To

Question -
Hi, maybe you can help me figured out something with my tires:

My vehicle comes with O.E. P215/80 R16 tires with a recommended tire
pressure of: 27 psi front and 30 psi rear, and I just change them to
LT255/70 R16 tires, so I want to know which would be the equivalents
front and rear pressures in order to maintain the load capacity correct,
and if this tires were P255/70 R16 too.

I know that this can be figured out from some YEAR BOOKS (TRA, etc.), but I don't
have access to them.

Thanks in advance

Best Regards.

Answer -
Daniel,

First I think you have the original tire size wrong and it's the "80" that doesn't make sense.

So check this and post back.  And just as a double check, let me know the year make and model of your vehicle and I'll look it up in Tire Guides.


Answer -
Daniel,

No need to apologize for your leaders.  I didn't vote for mine either!

And your explanation was fine.  I now completely understand where you are coming from.

Well....JATMA (Japanese Automobile Tire Manufacturers Association) lists a 215/80R16 (I didn't know they even existed!) and at 27 psi (185kPa) the load capacity is 752 kg and at 30 psi (210 kPa) the load capacity is 810 kg.  And just so we are complete for 35 psi (240 kPa) it's 875 kg.  BTW this tire has a Load Index of 103.

So for the equivalent load for a 255/70R16 LI 109.....well....it's off the bottom of the chart, except that at 180 kPa (26 psi) the load capacity is 870 kg.

BTW, JATMA follows Europe and doesn't use "P" in front of their passenger car tires, BUT! they use an "LT" BEHIND the size for those type of tires.

But because these are passenger car tires used in a Light Truck application, the load capacities are reduced by 10%.  I'm not going to show the math.

So an LT255/70R16 per TRA has an equivalent load capacity at 29 psi, 32 psi, and 35 psi.  (yes, I know that's 2 psi higher except for the last point, but that's what the table says!)

Hope that answers your question.

Answer -
Daniel,

Let me do this in chunks:

".....I don't understand why if my tires say that they can support a 1000 Kg. (2205 lb.) load at 50 psi, they need so little pressure to carry 752Kg.(@29psi) and 810 Kg.(@32psi)....."

That's because the begining of the load curve is very curved, where the area we generally work in is quite flat.  So while it is proportional once we reach a fairy low pressure the load curve drops off considerably.

"......We don't know what pressure would be the correct....."

In the days when passenger car tires were used on passenger cars and truck tires were used on trucks, there was a clear dividing line.  But the formula used for each is different.  In some respects it was based on different analyses and different assumptions.  Keep in mind, this was in the days long before computers, so all these analyses were done using a pencil and paper and to make it possible to figure this stuff out, a lot was based on what worked and what didn't.

Today the lines between passenger car tires and lught truck tires ovelaps considerably and the result is what appears to be entirely different answers (usage pressure) to the same question (load capacity).

So you need to keep in mind that the tires are tested according to the load tables - and even though they give different pressures for the same load, the constructions are different and are based on the load tables appropriate for each.

"......You always say that for P tires the minimum you recommend is 26 psi, so you can tell me wich is your recommendation on LT tires...."

This recommendation is based on the bead coming unseated during cornering.  LT tires ALWAYS use a higher pressure than 26 psi, so this isn't a problem.  

Answer
Daniel,

The load capacity of a JATMA 255/70R16 at 26 psi is 870 kg, but when used in an LT application, this would be 790 kg.

The pressure for an TRA LT255/70R16 where 790 kg is the load capacity is 35 psi.

I know this seems totally unreasonable, and I've struggled with it myself, but it is the way it is.

Ii has been a pleasure answering the interesting and knowledageable questions you have asked.  Don't hesitate to visit again.