Calling Capri Racer - inflation pressure on tire size change

[DriveHard]

Ok, here is the scenario.

Stock tires on my ZR2 are 265/65R17 with a load rating of 112S SL 2,469lbs @ 51 psi. The OEM suggests 35psi all around which I know lowers the load carrying capability of the tire...but to what?

I want to run summer tires 265/50R20 to reduce noise and un-sprung mass (yes, the rim and tire combo is lighter than stock). The load rating for the tire I am looking at is 111XL @ 51psi which I believe is around 2,400 lb capacity.

So...what pressure should I run the new tires at to keep the load capacity equal? I run the suggested 35 psi in the stock tires...I am thinking if I run 40 psi I would be safe. I know there is a way to calculate this, but I can't seem to find it.

 

[Pew]

@CapriRacer

 

[FowVay]

Just curious but do you intend to carry a heavier load with your new tire sizes? I too am curious if you should change your tire pressure from what the manufacturer recommends base on this tire size and load rating change.

 

[twX]

I've used the following formula, which seems to match load tables quite well.

Carrying Capacity = Load Rating * (P/Prating)^k, where k is ~0.48 for standard load P-metric tires, and ~0.75 for P-metric XL tires.

For your original tires at 35 psi, 2469*(35/41)^0.75 = 2193 lb
For the new tires at the same pressure, 2400*(35/41)^0.75 = 2131 lb
New tires with 36.3 psi, 2400*(36.3/41)^0.75 = 2186 lb

Edit: Fixed the calculation. Had used 51 psi for Prating instead of the 41 psi that should be used for XL tires.

 

[JHZR2]

I think this was already answered by capriracer sometime back, see the response here:

Always wondered why the tire Load/Index charts list 35psi max load?

The tire/index load charts have always listed the maximum load capacity for any given load index at 35 psi. That is fine for a tire rated 35psi max, but there are so many tires now that can take up to 51psi which should be able to carry more load, but the specs stop at 35psi not matter what size...

bobistheoilguy.com

 

[Kantaro]

Ok, here is the scenario.

Stock tires on my ZR2 are 265/65R17 with a load rating of 112S SL 2,469lbs @ 51 psi. The OEM suggests 35psi all around which I know lowers the load carrying capability of the tire...but to what?

I want to run summer tires 265/50R20 to reduce noise and un-sprung mass (yes, the rim and tire combo is lighter than stock). The load rating for the tire I am looking at is 111XL @ 51psi which I believe is around 2,400 lb capacity.

So...what pressure should I run the new tires at to keep the load capacity equal? I run the suggested 35 psi in the stock tires...I am thinking if I run 40 psi I would be safe. I know there is a way to calculate this, but I can't seem to find it.

https://www.toyotires.ca/sites/default/files/guide_application_of_load_inflation_tables_2020.pdf

112 SL @ 35 PSI = 2392 lbs
111 XL @ 42 PSI = 2403 lbs

 

[OVERKILL]

Ok, here is the scenario.

Stock tires on my ZR2 are 265/65R17 with a load rating of 112S SL 2,469lbs @ 51 psi. The OEM suggests 35psi all around which I know lowers the load carrying capability of the tire...but to what?

I want to run summer tires 265/50R20 to reduce noise and un-sprung mass (yes, the rim and tire combo is lighter than stock). The load rating for the tire I am looking at is 111XL @ 51psi which I believe is around 2,400 lb capacity.

So...what pressure should I run the new tires at to keep the load capacity equal? I run the suggested 35 psi in the stock tires...I am thinking if I run 40 psi I would be safe. I know there is a way to calculate this, but I can't seem to find it.

Assuming it's a P-Metric, you have to reduce it's capacity by a factor of 1.10 according to the Toyo handbook:
How to apply the load inflation tables (toyotires.ca)

So, for example, your stock tires are 2,337lbs @ 35psi, and then we derate and we get 2,125lbs

Now, the P265/50R20 has a capacity of 2,094lbs @35psi, unfortunately the Toyo PDF shows a 106 load index tire, so it doesn't list anything higher than 35psi. However, Falken had this to help us decipher:

So, 41psi gives us 2,359lbs, which, if we apply our derate, gives us 2,145lbs.

Conclusion: You should run 41psi in the new tires.

 

[Vercingetorix]

Tire Pressure Calculator

Easy to use tire pressure calculator. Use our tire pressure calculator to calculate the new tire pressure needed when changing tire sizes.

tiresize.com

 

[CapriRacer]

First, here's what you need to do this: TirePressure.org: Tire Load Tables

A couple of things: It doesn't matter if you're changing from P metric to Eurometric or vice versa - Choose one system. and stick with it. That's because there really isn't a difference between tires designed to one system vs the other. What you want to do is be consistent - so do it by size within one system or the other.

The other thing folks need to be aware of is that P metric Standard Load (SL) tires max out load-wise at 35 psi, but 41 psi for Extra Load (XL) tires - even if the max pressure is 44 or 51 psi. In Eurometric the max load occurs at 250 kPa (~= 36 psi) for SL tires, and 290 kPa (~= 42 psi) for Reinforced (XL).

The other thing you need to be aware of is that Eurometric tires are done in metric units (kg, kPa, Load Index), so English units (pounds, psi) are approximations. P metric tires are done in English units and metric units are approximations.

So the first thing I always do is verify the information given - and to my surprise GM used a Eurometric tire on the truck, but specified 35 psi. So the load carrying capacity of a 265/65R17 at 35 psi is 2400#.

Staying within the Eurometric system, a 265.50R20 111XL is max load 2403# @ 42 psi.

So there's your answer.

 

[jadatis]

Recomended pressure of yours 35 psi is not always calculated anymore for GAWR's and maximum technical carspeed with no reserve build in.
Often, as I suspect here, yust reference-pressure as already explained, that 35,36,41 or 42 psi.

If calculated it probably gives something like 25 psi for your car.

Will make you a list for old and new tires. Gives 90% of calculated axleloadcapacity for max speed 160kmph/99mph.calculated with an even tighter formula then the officially used in Europe (power ^0.8, twX contact me for detailed info) for all kind of tires, and since 2006 also for P-tires in SL and XL in US.
I determined this to give maximum reserve, but still acceptable comfort and gripp.

Then if calculated for real axleloads, only can be determined by weighing, estimating is dangerous, the new tires probably come to about 28 psi.
But you can use my list to search back 35 psi loadcapacity in old tires list, and search that loadcapacity back in new tires list for the new pressure,and find 43 psi.

Determining the axleloads 99% acurate is the most tricky part, and your responcibility

Here the list. In front old tires LI 112/ 2469 lbs AT 36 psi ( to be safe) used upto 99mph.
New tires LI 111 /2403 lbs AT 42 psi upto 99mph.

For higher pressure, I yust give calculated loadcapacity, but mind yourselfes not to use if higher then maxload of tire. Dont add reserves yourselfes, all done for you, yust read back the carefully determined axleloads.

Axleload/ cold psi/ Axleload new

2407 lbs/ 18,0 psi / 2036 lbs
2522 lbs/ 19,0 psi / 2134 lbs
2637 lbs/ 20,0 psi / 2231 lbs
2752 lbs/ 21,0 psi / 2328 lbs
2866 lbs/ 22,0 psi / 2425 lbs
2980 lbs/ 23,0 psi / 2521 lbs
3094 lbs/ 24,0 psi / 2618 lbs
3208 lbs/ 25,0 psi / 2714 lbs
3321 lbs/ 26,0 psi / 2810 lbs
3434 lbs/ 27,0 psi / 2906 lbs
3547 lbs/ 28,0 psi / 3001 lbs
3660 lbs/ 29,0 psi / 3097 lbs
3772 lbs/ 30,0 psi / 3192 lbs
3885 lbs/ 31,0 psi / 3287 lbs
3997 lbs/ 32,0 psi / 3382 lbs
4109 lbs/ 33,0 psi / 3477 lbs
4221 lbs/ 34,0 psi / 3572 lbs
4332 lbs/ 35,0 psi / 3666 lbs
4444 lbs/ 36,0 psi / 3761 lbs/ reference old
4555 lbs/ 37,0 psi / 3855 lbs
4666 lbs/ 38,0 psi / 3949 lbs
4777 lbs/ 39,0 psi / 4043 lbs
4888 lbs/ 40,0 psi / 4137 lbs
4999 lbs/ 41,0 psi / 4231 lbs
5110 lbs/ 42,0 psi / 4325 lbs/ reference new
5220 lbs/ 43,0 psi / 4418 lbs
5330 lbs/ 44,0 psi / 4512 lbs
5441 lbs/ 45,0 psi / 4605 lbs
5551 lbs/ 46,0 psi / 4699 lbs
5661 lbs/ 47,0 psi / 4792 lbs
5771 lbs/ 48,0 psi / 4885 lbs
5880 lbs/ 49,0 psi / 4978 lbs
5990 lbs/ 50,0 psi / 5071 lbs/ maximum allowed cold pressure both

 

[DriveHard]

Thank you @CapriRacer, exactly what I was looking for!

 

[930.engineering]

Stock tires on my ZR2 are 265/65R17............
I want to run summer tires 265/50R20 to reduce noise ............

Perhaps from a A/T tire to a touring summer tire? Yes, very likely.
But while keeping the same tire pattern? Never ever. Lower profile
will result in increased noise.
If you put emphasis on NVH I'd suggest to look for some type of a
tire that is known for superior ride comfort. Even better to just avoid
20" wheels.
-

 

[dumkid]

You can probably get away with running the oem pressure if you aren't maxing out your gvwr. If your tires stay cool (like 20f over ambient) they aren't severely underinflated. Correct me if I'm wrong.

 

[CapriRacer]

You can probably get away with running the oem pressure if you aren't maxing out your gvwr. If your tires stay cool (like 20f over ambient) they aren't severely underinflated. Correct me if I'm wrong.

Not so much wrong as much as more dangerous. In this case, that would be running the tires 7 psi underinflated!

 

[dumkid]

Not so much wrong as much as more dangerous. In this case, that would be running the tires 7 psi underinflated!

Yes they're underinflated but still higher pressure than the ford exploders and probably better tires (i know, the exploders are lighter etc.)

If these are low profile tires ignore me because the sidewalls already look challenged and I have no experience.

 

[jadatis]

Acording to my list, the notorious 26 psi of FF affaire, would cover 2810 lbs axleload for the new tires, with maximum reserve, at wich comfort and gripp still acceptable. Max speed 99mph, is a max speed you can control yourselfe, to stay below, I think.


So, topicstarter, what are your GAWR's