jcfay

Alright, related question then - anyone know how accurate the MB dash and TPMS system is in reporting actual tire pressure? I assume not so good, in which case my follow-up question is what pressure gauge do you like to manually check and adjust? Any favorites out there that are accurate and reliable? Thanks again

Ingenieur

the only way to know is to compare them to a good quality gauge
I use the $15 accutire digital, it's been test and accurate to 1/2 psi

Ingenieur

I noticed if you select tires with lower than oem LI on tire rack it gives you a warning and says you must increase pressure


eg, I was looking at snow tires and the selected front was lower than oem (iirc 93 vs. 95) so the pressure must be set at 42 psi (higher than the oem max of 41) to achieve an equivalent load capacity

Diabolis

38 psi may be OK with 18" wheels, but it's far too low for 19s. Aside from pressure requirements to achieve the proper LI foir a certain weight, you also need to consider the height of the sidewall. Unless you have at least 44 psi in the fronts, you'll cut the tire and/or bend the rim on the first pothole. Plus, I don't know that I'd want to attempt any high speed runs with tire pressures that are too low. With 19s, you probably want to run 45 psi F/R.

Diabolis

The dash only reads what the sensors report.

On two sets of wheels that I have, all four TPMS sensors are dead-on. On the third set, three of them are dead-on and one sensor is reading 1 psi lower than actual (compared to a calibrated gauge accurate to within 0.5 psi).

Ingenieur

With oem tires I would not run lower than the minimum on the gas flap

jcfay

Alright, again I'm confused. I understand that one needs to take into consideration the lower sidewall height of a 275/30 versus OEM 255/35, for example. So we don't only use load index then to determine pressures? I've switched my fronts from the OEM 18" Conti 235/40 95Y to Hankook Ventus V12 Evo2 19" 245/35 93Y, and my OEM 18" Conti 255/35 94Y rears to 275/30 96Y rears. Since I'm most often solo in the car I was running my OEM 18" at about 38 or 39, front and rear. Since 1 load index goes up, and another goes down, what the heck pressures should I be running? But I'm also running smaller sidewalls too...

D C

Dear god. Worrying about the load index vs. tire pressures of your tires is worse than worrying about whether you have the correct temp spark plug for your car.

Unless you are constantly loading your car with a ton of weight (literally), your tire pressures don't matter in that regard. Set your pressures so you get good wear out of them which is normally around 36-40psi.

The decals on the car don't mean jack. The MFG can put what ever they like on there, which can be contrary to what the tire MFG recommends. Anyone remember the Ford/Firestone fiasco? That was because Ford put very low recommended pressures on the Explorer so it would ride better.

jcfay

OK, that's pretty damn vague. But that's cool if that works for you. I'm assuming there probably is a pressure range (perhaps 36-40 as you suggest) that's appropriate, and I'm (obviously) the overly analytical type of person that looks for exact answers. It can be a bit trying... You use 36-40 for 18" and 19" wheels?

D C

There are NO exact answers. Set your pressures somewhere between those ranges and your good to go for daily driving. On ANY tire, on ANY size rim.
Look at your wear after a few thousand miles. In general, more wear on the shoulders means you don't have enough pressure in them. More wear on the middle of the tire, you've got too much pressure.
Alignment settings and your driving habits will play a part as well.
Your tire pressures will probably vary more than that on any given day anyway depending on if it's hot or cold out, and how your driving.

For myself while DDing the car, I'll set them to 38-40. On ANY tire, on ANY size rim. For max performance/grip, that's a whole 'nother can of worms.

Quit worrying about the little stuff and enjoy the car.

Ingenieur

that is dangerous advice
if you replace a tire with a lower LI than oem and run the light load oem pressure setting you are increasing the risk of failure

no one is 'worrying' about it mr. care free
do not confuse discussion with 'worry', you are projecting
a tire failure at 100 mph is not 'little stuff'
tire (along with brakes & steering) are important safety equipment, a little thought (or worry as you call it, lol) is in order


a 3500 lb 2 passenger coupe does not require the same pressure as a 4500 large sedan
the 38-40 psi ON ANY TIRE is absurd at best, dangerous at worse

Ingenieur

the short version
if oem spec, LI, etc., run the mfgs recommended values
they didn't pull them out of their @$$

if you have increased the LI
you can run oem values but will ride hard and wear the middle out
you can run lower
from the load tables
look up oem pressure for the oem LI: it will give you a load in pounds
find the new LI and using the pounds from above find the corresponding pressure, use that

if you have a lower LI than oem, it becomes more important to use the charts and raise the pressure...I do recommend ever using a LOWER LI than oem

believe it or not: there is a reason the tire mfgs and standards orgs publish the load charts lol

jcfay

Do you mean this load table (I think you linked this earlier)?
http://www.tiresafety.com/images/Tir...t%20Manual.pdf
This isn't brand specific?

jadatis

The reason for giving low Aspect ratio tire ( low hight/width division yours 30 and 40 %) a higher pressure advice even if LI is higher is next.
I found out that low AR tires are given to high maximum load by the tire-makers.
Got a graphichs in wich deflection of several AR tires from 80% to 30% with reference/maxload-pressure in it and maximum load on it.
Then asuming the 80% AR tire with 20% deflection of the free flexible part of the sidewall is save so leaves the tire undamaged when driving 99m/h, the 30% AR tire had about 27% deflection.
Calculated this back what the maximum load would be when 20% deflection roughly , and came to the conclusion that 20% of maximum load should be substracted of such a tire.
Confronted the deliverer of the graphics with it and even concluded that such a 30% AR tire may only have about 17% deflection ,to give it the same bending of the rubber at places , so same heat-production wich it can cool down sufficiëntly.

To make it short , substract of lower then 45% AR tires 20% of the maximum load, or take 8 LI steps lower, before putting them in my calculators to determine the pressure for lower loads. Its a rough calculation so in real it can be less or even more to substract.

And that is the reason why you need that extra information wich seems to make it all complicated. Because of the inadequate universal calculation the tire makers use all over the world.

Autosport7

LI's - Load Index...?, AR's - Aspect Ratios....?
Sorry if it was mentioned but what does all this mean?
I am obviously clueless to this subject, but sounds more important than I invisioned.
Can I get some cliffs by chance?
I am running 235/35/18 and 265/30/18. Can I ask the pros what pressures I should be running at cold tire temps (not driven on yet)? Or do you need more info from me like make & model?

Ingenieur

No
All tires are rated the same
A Conti 95y is the same as a Michelin 95y
It is a common rating system agreed to by all mfgs

D C

Taken from the bottom on EVERY page in section "B" of http://www.tiresafety.com/images/Tir...t%20Manual.pdf
NOTE A: Some P-Metric Light Load and Standard Load tires may be branded with a maximum inflation pressure of 35, 44 or 51 psi for high speed and handling optimization. However, there is no load increase above 35 psi.
NOTE D: Some European Metric Extra Load tires may be branded with a maximum inflation pressure of 42 or 51 psi for high speed and handling optimization. However, there is no load increase above 42 psi.


OMGWTFBBQ!!! Max load capacity of ALL these tires happens about 36-40PSI. Amazing.

Ingenieur

LI = load index
The numbers below are made up and used for illustration clarity
The mfg specs a 93 LI at 36 psi for the load it determined the car to be
You look in the table and 93 and 36 psi is rated for 1000 lbs

You buy a wider tire with a 94 LI
When you look at the tables you see this tire only needs 34 psi for the 1000 lb rated load
This will allow proper contact patch, better ride and better wear

If you by a 92 you see from the table to carry the rated 1000 lb the pressure needs increased to 38 psi
If not handling will suffer, tires will deform and overheat, they will ride on the edges and wear poorly
Ride comfort may suffer too

jcfay

At the very least I get some pleasure knowing that I may have provided you with the fodder to support your argument. I noticed that footnote earlier today too, so will be running my fronts at 42 and my rears at 39. Note, I'm not running them at 36 all around, or 36 and 40, or WTF you run yours at. I'd like to avoid destroying my wheels that cost more than many people's cars. While I realize tone is very often lost in forum posts, your tone seems a bit snarky. I will, however, go BBQ as I'm getting hungry.

I'm tempted to ask you what motor oil you're running as there have been INSANE debates on that subject, but will refrain. Do you run 229.5 or 229.51 spec? Just kidding. And I'm enjoying the drive plenty, thanks. I love this frigging car

Ingenieur

Load rating does not increase above a certain point
But most pressure specs are below that point

C63 front
OEM 235/40/18 XL 95Y light load 39 psi 1433 lbs

Upsize 245/40/18 XL 97Y takes only 36 psi for 1433 lbs

3 psi is significant, almost 8%

Ingenieur

The load rating/pressures are not only static loading
Low load (3 people plus luggage) spec for my car is 39/39
For OEM tires that is 2 x 1433 + 2 x 1400 = 5666 lbs
The actual dead load is closer to 3950 + 3 x 175 + 150 = 4625 lbs
1000 less than the tire rating
This is 20% for accel, cornering and impulse forces

D C

Snarky, possibly, though without intending it. I also don't have kids, so I haven't learned enough about patience. That may have contributed.
Since coming to this forum I see alot of discussion about "little things" and greatly overthinking them. In the motorsports world, I equate this to arguing about something that would affect your time by fractions of a second, yet your driving skill isn't anywhere near the ability to see that difference.
Oil discussion - I've been beaten into submission. Actually, Ingenieur has shown me the light, or at least the path to it, so between that and other research into oils, I now see the merit in sticking to the approved oils. So far, mines been changed at the dealer. FWIW, as I've seen mentioned elsewhere, 229.51 was specifically created for diesel motors. So 229.5 for us.

And this further proves my point. Even if you set your pressures to MY minimum of 36psi, your still in a very safe pressure range that doesn't significantly reduce your load rating.

Ingenieur

It actually proves your point is wrong and dangerous

If you set a 91 LI to 36 it will likely fail if the vehicle is loaded
Heat up and fail or roll off the rim when high speed cornering
There is a reason the auto mfg gives settings for various loads
And there is a reason the tire mfg goes to the trouble of developing load charts

Set yours where you like
But don't encourage others to follow your baseless and foolish advice
It is dangerous

Btw 36 is BELOW MB's minimum pressures
Advice like that is a liability

Ingenieur

OEM front tire
36 psi 1356 lbs
GVW Front 1350 lbs per side
Fully loaded ~ 4900 lbs x 0.55 /2

You'll be fine as long as you dont drive the car
Just sitting there hunky dory lol
You need 10-20% more for accel, cornering, bumps
MB recommends 41 psi load rating 1477
Or a 10% margin, marginal but safe

D C

OK, if you say so, but that is contrary to the linked info. A 91 LI load is 1356lbs, achieved at 36psi.