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Old 01-17-2016, 04:44 PM
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let's talk tyre pressure

Good morning all,

Just got back from my local (and reputable) tyre place. I had a slow leak in my rear left tyre which turned out to be a 3 inch nail.

My tyres are the Michelin Latitudes that are stock on the Laredo's

$33 dollars (premium Sutherland Shire charge included) later i was on my way, but not before the good lads at the tyre shop informed me that my tyre pressure all round was way to low. I said, it is set to spec of 32psi.

He said, DO NOT UNDER ANY CIRCUMSTANCES listen to the manufacturers specs for tyres. He said anyone with these tyres needs to be at 40psi across the board (unless offroad of course).

He also said, that by being at the spec setting i have cost myself 10,000kms of tyre life and that the tyres were not wearing evenly across the whole car.

This is not the first time i have heard this, and this is not nearly surprising but i still always have kept it at the Jeep spec because at the end of the day you would expect they know the car best. But if a professional and reputable tyre place is telling me the opposite it intrigues me.

So what do you all keep your tyres at? What are your thoughts?

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  #2  
Old 01-17-2016, 05:09 PM
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Re: let's talk tyre pressure

There have been so many articles written about tyre pressures over the years. From what I have read, the consensus appears to be to stay with the manufacturers recommendations. Those recommendations are supposed to be based on Comfort, Fuel economy, and Endurance. Go figure?
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Old 01-17-2016, 05:33 PM
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Re: let's talk tyre pressure

The sacrificial part of that the tyre place is allowing to be removed, is comfort. A major part of the manufactures pressure spec is for small bump comfort. So upping the pressure removes that at the addition of grip on regular roads, and tyre life overall. My Falcon has a recommended about 32/34. I run 39/40 in it for the same reason.
I haven’t played with the GCs, I don’t spend enough time in it to be able to tweak it accurately.
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Old 01-17-2016, 05:39 PM
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Re: let's talk tyre pressure

I run both my 18's and my 20's at 40psi.
Always have, always will.

The small bump discomfort going from 32 to 40 would be imperceptable in the real world but the increased tyre life, better rolling resistance (hence better economy, even marginal), better handling (tyre less likely to roll onto the sidewall) all help.

IMHO........
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Old 01-17-2016, 07:18 PM
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Re: let's talk tyre pressure

I run 36psi at cold, which is 40 when warm. But that is on LT tyres.

32psi on passenger tyres does sound low. They will be able to tell if the pressure is too low by the wear, it will be more obvious on the shoulders of the tyre, over inflation will have more wear in the middle of the tread.

I use the 4 psi rule. Inflate, then drive for an hour and check the pressure. If it is 4psi higher when warm, then it is right. If it is less than 4psi, then pressure is too high. If more than 4psi, then pressure is too low. But you need to be careful, because obviously ambient temps can play a big part here, and then if you are later adding load then pressure requirements may change.

Wear:

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Old 01-17-2016, 09:21 PM
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Re: let's talk tyre pressure

I have read of the 4% rule before, but always thought it was deeply flawed as ambient temperature changes alone can cause that type of fluctuation before heat from actual driving is even applied.

Edit: I wrote "floored" instead of "flawed", spanner.
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Old 01-17-2016, 10:07 PM
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Re: let's talk tyre pressure

I had a debate with the Coopers rep about the 4 psi rule, from 18 psi all the way to 45 psi my tyres increased by the same pressure on the same day. This wasn't 4 psi either.

Hotter day and they increased more.

It's a misunderstood rule of thumb which is barely applicable today.
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Old 01-18-2016, 03:46 AM
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Re: let's talk tyre pressure

I've had consistent results using the 4psi rule, from memory I tried 40, and they went to 42 hot. And when I tried low 30 they went up 6 or 7 psi.

As I said, you need to be careful with ambient temp changes, start and finish in similar ambient temps.
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Old 01-18-2016, 06:09 AM
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Re: let's talk tyre pressure

Tyrepessure advice is determined using the expected weight on seperate tyres, maximum speed you wont go over for even a minute, and sometimes alighnment ( camber angle wheels like this on the imaginary axle /-\).

The car maker has to give an advice for extreme situations so he never can be held responcible when someone for instance overloads or drives faster then allowed.

But this means that the carmakers advice is high enaugh, at least for European cars. In America they used a formula before 2006 that lead to to low pressure so to much deflection so to much heatproduction wich lead to tire-damage.

If you give me weights determined as accurate as possible, and speed you wont go over for even a minute, and of tyres maximum load or loadindex, speedcode, and kind of tyre to determine the maxloadpressure( wich is not the maximum allowed cold pressure of a tyre as given on sidewall of Standard load and XL/reinforced/Extraload tyres), I am able to calculate a save lowest pressure for the situstion.

If more situations wanted, fi when towing or fully loaded, give speeds and loads for that and I will calculate different advices.
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Old 01-18-2016, 06:31 AM
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Re: let's talk tyre pressure

Quote:
Originally Posted by Benn0 View Post
I've had consistent results using the 4psi rule, from memory I tried 40, and they went to 42 hot. And when I tried low 30 they went up 6 or 7 psi.

As I said, you need to be careful with ambient temp changes, start and finish in similar ambient temps.
You are measuring using your TPMS yeah?

Interestingly, you will find many people calling for LT tyres to use the 6 psi rule instead.

(Can google for this)..
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Old 01-18-2016, 10:17 AM
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Re: let's talk tyre pressure

Quote:
Originally Posted by SnakeDoctor View Post
You are measuring using your TPMS yeah?

Interestingly, you will find many people calling for LT tyres to use the 6 psi rule instead.

(Can google for this)..
Better would be to go from a percentage pressure rising of 10 to 11% .
Or even better is to monitor tyre temperature .
Average normal warm inside tyre temp when driving about 60 m/h is 45degr C/110 degr F.
This is when ambiŽnt temp is 18degrC/65degrF. So better is 27degrC/45 degr F above ambiŽnt.
Verry high ambiŽnt temp the temperature rising must be a bit less .
If warmer , put higher pressure cold to give lesser deflection so heatproduction.

Will give pictures I made of pressure rising for temprising in degr F/psi and degrC/kPa ( 100 kpa=1 barr=14.5 psi)
Attached Images
File Type: png psichangeforfahrenheit5to90psiadvice..PNG (35.1 KB, 19 views)
File Type: png kPachangebytempchangedegrC50to360.PNG (35.7 KB, 13 views)
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Old 01-18-2016, 02:49 PM
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Re: let's talk tyre pressure

I also use the 4 psi rule, works for me.


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