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Old 22-05-2014
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I will add in my stats. D load rated 285/70 17's or metric 33's as some refer to them on a short wheelbase JK I run at 28-30 psi on road after much trial and error for even wear and comfort. Dropping to around 18-20 on sand or off road. If towing the van which adds another 150kg to the back end I up the rears to about 34 on road and about 24 off road or sand.

After picking it up from a wheel balance on Monday I discovered the tyre shop had set them all at 44psi... When I questioned them they informed me (or misinformed me is probably more accurate) that the tyres being larger than stock needed to be much higher pressure than stock. Yes it would get marginally better fuel economy but as the edge of the tyres barely touch the ground the centre of the tread would have worn out in half the distance and the ride quality was shocking. Unbelievable what some so called experts sprout out as facts!

Sent from a tiny keyboard with fat fingers, apologies for my spelling. .
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Old 24-05-2014
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Your right Dave, most tyre shops have no idea. I worked in them for years & saw first hand how wrong they can be. Every vehicle requires a certain volume of air to support its weight. If you change to a larger tyre, you require less pressure to obtain the same volume & if you change to a larger diameter rim & a lower profile tyre, then the pressure must be increased to obtain the same volume of air. It's not rocket science...
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Old 24-05-2014
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Correct.
Volume pressure and weight all effect the other.
The forumla isn't "accurate" but basically Vol x Pressure = weight. Vary accordingly.

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Old 24-05-2014
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Then you throw in LT vs passenger tyres with stiffer sidewalls on the LT, and then different sidewall strengths of LT (D rated, E rated etc) and the calculations get thrown kut the window and yoy back to trial and error with an educated guess as the starting point!

Sent from a tiny keyboard with fat fingers, apologies for my spelling. .
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Old 24-05-2014
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Quote:
Originally Posted by SeaComms View Post
Then you throw in LT vs passenger tyres with stiffer sidewalls on the LT, and then different sidewall strengths of LT (D rated, E rated etc) and the calculations get thrown kut the window and yoy back to trial and error with an educated guess as the starting point!

Sent from a tiny keyboard with fat fingers, apologies for my spelling. .
I couldn't agree more with this.
Trying to use formulas and exact calculations to operate in volatile terrains is pretty much a waste of time.
All you really need to know is that you need to let the tyres down. The terrain can change from one day to the next.
Sand is a perfect example. The change in humidity can decide which pressure you need and even which gear range you need to select within just a few hours!
There is currently a "which range should I select?" thread happening elsewhere. It's the same for that.
If progress is impeded or your vehicles hurting, you adjust accordingly.
I would also add, this is why we are out there, to test, try, experiment and have fun doing it.
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Old 24-05-2014
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Quote:
Originally Posted by SeaComms View Post
I will add in my stats. D load rated 285/70 17's or metric 33's as some refer to them on a short wheelbase JK I run at 28-30 psi on road after much trial and error for even wear and comfort. Dropping to around 18-20 on sand or off road. If towing the van which adds another 150kg to the back end I up the rears to about 34 on road and about 24 off road or sand.

After picking it up from a wheel balance on Monday I discovered the tyre shop had set them all at 44psi... When I questioned them they informed me (or misinformed me is probably more accurate) that the tyres being larger than stock needed to be much higher pressure than stock. Yes it would get marginally better fuel economy but as the edge of the tyres barely touch the ground the centre of the tread would have worn out in half the distance and the ride quality was shocking. Unbelievable what some so called experts sprout out as facts!

Sent from a tiny keyboard with fat fingers, apologies for my spelling. .
Mate, about 2 weeks ago I got some AEV rims and BFG MK2's in 285/70r17.
They were set at 46psi. OMG just amazing. So now they are at 30psi.
Last week I did the 4psi road test.
I am not sure if this theory is any good or not.
But cold setting at 30psi and after a 40-45min run I checked and they were at 34psi.
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Last edited by Aus Patriot; 24-05-2014 at 04:34 PM.
  #14  
Old 28-05-2014
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Quote:
Originally Posted by SeaComms View Post
Then you throw in LT vs passenger tyres with stiffer sidewalls on the LT, and then different sidewall strengths of LT (D rated, E rated etc) and the calculations get thrown kut the window and yoy back to trial and error with an educated guess as the starting point!

Sent from a tiny keyboard with fat fingers, apologies for my spelling. .
The "formula" isn't out the window - as I said it's not for exactly "measuring" the required pressure.
But just helps explain why you need to increase/ decrease pressures as you vary one of those elements.
I agree that construction variances are also just one of the other variances, however the same basic principles apply.
Problem with a lot of tyre shops is the apprentise looks at the sidewall and sees "max loading 44psi at 950kg" and thinks ok the tyre needs 44 psi, however that's only if the vehicle is currently weighing 3800kg....
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