Ingenieur

Finally found a load chart
oem
235/40/18 95Y xl 41 psi = 1477 lbs
255/35/18 94Y xl 45 psi = 1477 lbs
(42-45 psi = 1477 lbs)

My sizes
245/40/18 97Y xl 41 psi = 1565 lbs
265/35/18 97Y xl 45 psi = 1565 lbs

So imo using the oem pressures in plus size applications is not right
The pressures should be adjusted so the load ratings for both sizes are matched

245/40/18 97Y xl 38 psi = 1477 lbs
265/35/18 97Y xl 36 psi = 1433 lbs

I'll probably use F/R of 38/38

This should help wear, handling and ride comfort

nkc

you are correct, after playing with every pressure there is in my OCD state, i agree with you 100%

i run 39/42 for my 235/35 & 265/30 19's PSS, will be getting RE-11 245/35 & 275/30 and i intend to test and see if i will go a tad lower considering the sidewall on re-11 is rock solid as against the PSS that flex's, will see how that goes in a few days time.....

donaldt

on PSS and I have the Optional 19s, I tried going from 36 to 42 front and rear
I settled for 42 front and 40 rear as it handles better and turns quicker than 36
not scientific at all but I thought it makes more sense for the super heavy nose to have more support?

Ingenieur

Tire pressures sre selected primarily on load
I used the loads MB used to set their pressues for the oem spec tires
I then looked up the pressure for same loads for my tires
F/R
Oem 39/39 = my 36/36
Oem 41/45 = my 38/38

I set them 38/39
After driving them a while I may adjust them to 37/38 or 37/37

I am Jeff

I don't know how the roads are out by you, but they are crap in NYC. If your roads aren't smooth, my suggested settings in terms of comfort and wheel protection is 39/41.

Ingenieur

roads aren't bad but do have their rough spots
but even at a lower pressure mine should afford the same protection since they are rated for the same load

load rating oem 95/94, mine are 97/97...so for a lower pressure I can carry the same load as oem does at a higher pressure, so the load the tire sees is the same... in fact my settings of 38/39 are a higher load rating than oem 41/45 (which is full loading)

after driving some today the car feels much better
smoother/quieter, better ride
better grip in the X axis (straight)
seems to be the same in the Z (lateral)

I put some chalk on them and it wore evenly
with the higher pressures it loaded more in the center of the tire

might drop to 37/37 which would be basically in the middle of oem light to heavy loading...but I will drive as is for a while
the only thing that may suffer a bit is mpg since I put more rubber on the road

I am Jeff

At the lower PSI, I had a mishap last year that required me to rebarrel one of my HRE's. Thank god for the forged centers, but for me, the higher PSI affords me better protection and peace of mind. I'm also running 275's rears.

Diabolis

Using OEM pressures in plus size applications is definitely not right. All other things being equal, 17" wheels usually require around 36 PSI, 18s require around 40 and 19" require 44. You roughly need a 4 PSI pressure increase from the OEMs for going up one size.


The load rating depends on the construction of the tire and is calculated at a specific pressue (42 PSI IIRC on the 2010 spec), but it doesn't proportionally decrease as a function of the actual pressure in the tires. Running a lower pressure only results in having less tire beween the road and the rim (so potholes are that much more likely to actually make contact with the rim, and in more heat being generated by the tires themselves as a result of the increased sidewall flexing.

Ingenieur

MB selects a pressure for a specific tire based on that tires load spec and what they calculate the load on the tire will be

My tires have a higher rating (95 vs 97) so can be set at a lower pressure for the same load rating

That is the sole reason they make load vs pressure tables

Jeff what is your 275 spec??? Size, Load rating and is it an XL???
I dont think the 275 is made in a xl

I can tell you what the book says for a MB Equivilent load rating

The only way to set pressure in other than an oem fitment is to use the tables
My tires at the lower pressure afford the same load capacity and rim protection as oem at the higher pressure

Table: http://www.tiresafety.com/images/Tir...t%20Manual.pdf

For example
A 235/40/18 95Y xl at 41 psi = 1477 lbs (MB 's rated load and pressure)
And a 245/40/18 97Y xl at 41 psi = 1565 lbs

So the 245 can have the pressure lowered unitl the rated load is 1477
That is 38 psi from the tables
No need to have it higher since MB says the load is 1477

I am Jeff

My specs are XL rated 96Y. I'm currently using 275/30/19 Goodyear Eagle F1's for my rears.

http://www.tirerack.com/tires/tires....omCompare1=yes

Ingenieur

Oem spec
39 psi 1400 lb light load
45 psi 1477 lb heavy load

Your tire
37 psi 1411 lb light load (36 psi is 1389 lb)
39 psi 1477 lb heavy load

You could run 39 psi and achieve oem full load
I'd run 38 psi which is 1433, right in the middle of light/heavy loads

jadatis

First is how did you came by the maximum loads to the Loadindexes of 94/95/97.
then are the 41/45 psi the pressures you read from the sidewall as maximum or are they the advices pressures on the car.

Then low aspect ratio tires ( hight/width division yours 40/45 are given to high maximum load ( so Loadindex) on the sidewall to laws of nature.
Best is to substract 20 % of given maximum load or take 8LI steps lower.
then after that you can use my made pressure/loadcapacity lists that can be found here.
https://onedrive.live.com/?cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%21904
Open the USA map and search the right psi so for XL/reinforced/Extraload is 41 psi but exeption to 44 psi is possible and mayby yours given 45 = 3.1 bar is new to me but can be the pressure needed for maximum load and not the maximum pressure of tire.

then if you found the right list try to determine the actual loads on the tires as acurate as possible and add up to 10 % for reserve.

Ingenieur

MB set the loads based on tire spec and pressure
Actually they picked the psi based on tire spec and loading
This can be read from the tables

The 41/45 are the full load spec for my car from the filler flap
39/39 light load
Should have clarified this vsries with MY And body type

The chart gives the load rating for various psi for every standard tire spec im production

jadatis

So to see if I understand right ( I am Dutch so can misinterprate some terms) .
You have for your car pressure loadcapacity lists for different possible tires for your car , given by MB in manual or something.
then the loads are given for the pressure ar probably right if the lists are from after 2006 , but to high of the lower pressures if lists are from before 2006. American TRA stepped over in 2006 to the calculation of the European ETRTO.

Ingenieur

GVW 4872 lb
Max per axle 2469
From door sticker

Each corner fully loaded max 2469/2 = 1234.5 lb
The usual safety factor of 1.2 is applied for impulse and g force
=1481 lbs very close to the 1477 lb that the MB full load pressures equate to on the load table for oem spec tires
The difference is rounding/ conversion for kg to lb and bar to psi

Ingenieur

The lists or charts are made by the tire manufacturers organization in collaboration with IOS, DIN, US DOT, etc not MB

The charts are dated after 2006

MB gives the pressures for light (39/39 psi) and heavy loads (41/45 psi)
They also spec the tire, in this case
235/40/18 95Y XL front and 255/35/18 94Y XL rear
from the chart this can be converted to a load in lbs ( using the heavy load)
Front 41 psi 1477 lbs
Rear 45 psi 1477 lbs

For a different spec rear tire
265/35/18 97Y XL
Looking in the chart for 1477 lb and read off the associated pressure
In this case 38 psi

Summary
A 265/35/18 97y xl at 38 psi will safely carry the SAME LOAD as a 255/35/18 94y xl at 45 psi

Ingenieur

I think we are saying the same thing

If you substitute a tire with a different LI than oem
If the new LI is lower than the oem one you must increase pressure
If the new one is higher you can air down

The tables tell you where to set the new LI tires pressure to achieve the same load capacity

Too high is bad for wear
Too low is dangerous
You want the sweet spot

jadatis

Missed internet on my vacation for a few days so could not react.
But your tires are probably from the European system because I googled them and found on duch site also the 94 Loadindex for the 255s , American system uses lower pressure for wich they calculate the maximum load ( so loadindex) , for XL thats 2,8bar/41 psi and europe 2,9bar/42 psi wich leads mostly to 1 LI step lower for American system then European.

Why then that 45 psi , thats for higher speed and camber angle above 2 degrees.
I have that system they use for that and also the formula European ETRTO uses and since 2006 also American TRA uses for normal car tires ( P-tires) and XL/reinforced/Extraload.
Wil calculate for you if I have more time , but the advices on sticker are given for maximum speed of car and GAWR's are filled in so fully loaded, wich 99% of use is not needed.
Rear lately ( after 2000) often maximum pressure of tire is given as advice for fully loaded, and that can also be 45 psi so 310kpa/3.1 bar.

So try to find maximum technical car speed , Empty weight of car, and if you use KM or Miles per hour.

Ingenieur

here's the issue

a tire with a LI of 97 does not require the same pressure as one with a LI of 95
it can be lower for the same loading (car weight and passengers/baggage) and speed
this will result in better ride, wear and traction

top speed is limited to 155 MPH, car weight loaded is 4872 lbs, MPH
load index is 97 for the front and rear (vs. oem of 95/94)

jcfay

Alright, I couldn't be more confused. What then should determine if we use "heavy" or "light" load settings? I ain't pulling no trailers but do drive a bit fast sometimes... ;-). I mean how many friggin variable are there to consider? I'm moving up to 19" Klassen M52Rs, with a 245 front and 265 or 275 rear (standard profile, I forget the #s), I can't recall tire manufacturer now and have no idea what load rating of tire is yet. I drive a '14 coupe.

I guess I've got some more research to do...

Ingenieur

light: 3 passengers + baggage
heavy: 5 + baggage

I set mine in the middle

you need the LI (load index) to set them properly

jadatis

My idea is that your 39/39 for light load is realy for 4 persons and load so GAWR s are used as load .
Before 2000 normal use /light load was for a load the car maker determined for 3 persons and a little load. After 2000 it was for GAWR's filled in and that 4 persons called.

What the formula means to calculate is the lowest pressure for the load and speed ( and camber angle) that dont give damage to the tire by the repetative bending of every segment of the tire. What you do above that is for handling reasons.
And even you can influence over- or Under-steer with it.

the empty weight of the car can also be determined by substacting the payload from the GVWR ( gross vehicle weight rating) the 4872 lbs of Ingenieur.

Most think that these advice pressures are determined by severe testing , but they are yust calculated for the load and speed, so for savety of the tires only.

Ingenieur

The car label pictogram show 3 people + luggage for 39/39
5 for 41/45

You are correct
The pressure setting is based on weight and the LI based on tire mfgs standards
That is why the pressure can be lowered for a higher LI for the same car/load
It can be increased for sustained high speed driving
But the load capacity is not increased
Not eally an issue in the States

Gvw 4872
Max load 848
Curb 4024

I'm running 37/38 now and it feels great
May go to 36/37
My LI are 97/97

jadatis

I dusted off an older spreadsheet of mine for recalculating tire pressure.
Had to load it from my one-drive wich you can find navigating from the link I already gave. But came in a bit damaged so I think rounds instead of roundup.

It uses the european ETRTO calculation and system .
First lowered the 97 Loadindex by 8, so 89/580kg/1279lbs, to give the tire a deflection that is save for up to 99m/h,to laws of nature and used the 42 psi reference-pressure and not the American 41. Then this 1279 is yust enaugh to cover your axle loads of 2469lbs/1120kg.
Then it has to be rounded up to 3.2 bar/47 psi for speed of 155m/249km/h.

Determined your axle loads empty at 4024/2= 2012 but motor in front so
estimated 2124 front and 1900 rear and 2 persons of 75kg/165lbs ( so correct it if its different) on the front seats so more then half on front and rest on rear axle. Gives 2124+180= 2304 on front axle and 1900+ 150= 2050 rear axle. Assumed this to be your 99% of use.
Filled that in my calculator as your own given weights.

then the advice pressures are for your ( my asumed) normal use.
up to 137m.h** 37psi front and 32 psi* rear. for vacation use so a little overloaded on rear ( 5 persons of 75kg/165lbs= 825lbs/375kg so yust under your payload wich means that in fact your rear axle will be overloaded.)
137m/h** Front 41 and rear 43psi.

* Rear I did not include a possible camber angle of above 2 degrees ( wheels like this on the ( imaginary) axle /-\) but low Hight/width division tires like yours 40/35 are more sensitive to that) for 4 degrees the official system gives 15% higher pressure needed)
Also its better to add 10% to the weights for reserve for , pressure loss in time, misreading/yudging of weight and pressure-scale, unequall loading R/L, etc. Filled that in too as your own given weight and it gave F42/R36.

** the 137m/220km/h is what the tire maker gives as speed for Y speedrated, to be the speed for wich the 42 psi is used to calculate the maximum load.
But for up to V speedrated that speed is 160km/99m/h and I suspect the tire makers to have calculated the maximum load also for that speed for Y .
for W speedrated is the other exeption and is 190km/118m/h and I have seen W and V speedrated tires of same sises and XL that have the same maximum load, so suspect them to calculated by mistake for also 99m/h the W and Y speedrated.
So that is why this advice is better to use for up to 160km/99m/h, wich is to my opinion also a speed you can hold yourselves to, even if you drive over the speedlimit sometimes.

At these advice pressures you dont have to worry for the speed and loads they are given for, that your tires bend to much and so get damaged by the 10 to 20 times bending of every segment of the rubber every second wich produces energy in form of heat.
If any spot of the tire gets above about 250degr Fahrenheit/ 130 degr C Sulfur bridges are getting formed wich make the rubber hard so not flexible and strain forces make crackes at those spots.

so probably the advices rear have to be highened up by 15% for camber angle ( but an angle of the road for instance riding in a truck track has to be added to that) .

then every thing you do above that is for handling reasons.
For instance rear higher pressure gives less understeer or more over-steer.
Higher pressure front gives lesser over-steer of more under-steer.

So now enaugh from this self asighned tire-pressure - specialist.
If you want to know more, for instance the copy I have of the calculation and system of ETRTO , wich started my tire-pressure story, Mail me at my hotmail.com adress with username jadatis, and I will sent you it in return.
For possible copyright reasons I have not placed it public.

JRHolt

I have been running 38 PSI front and rear and it rides great.

The dealer had 50 psi in all of the tires!!! OMG it rode like crap!

Not sure about wear yet, only have 1900 miles on it so far