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Tyre pressure - what is the correct pressure?

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In a recent thread about fuel economy the comment was made about checking that tyre presures are correct. No argument with that, but it set me wondering as to what are the correct pressures when non OEM tyres are fitted. Many of us have "non-standard" tyres, and correct inflation pressure is a safety issue, as well as affecting fuel economy and tyre wear. On my 110 CSW, in summer I run on General Grabber AT2 265/75/R16 and in winter on Michelin Alpin 205/80/R16. I generally inflate to 32psi front and 38psi rear, which seem to be OK, but to be honest, I have no idea what the pressures should be for each set of tyres. Are manufacturers' tables of inflation pressure vs. axle load available anywhere? They must have these, since tyres are stamped with maximum inflation pressure and maximum load.

Has anybody been down this road, and can anybody, please, advise me how I can determine the correct inflation pressure for each set of tyres on each axle, and, of course, at different load conditions? Many thanks for your experience, thoughts and views, in advance.

Mike

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You can work out the 'correct' pressure for your tyres with a piece of chalk.....

Draw a line across the tyre then drive a very small amount and check the chalk wears off evenly, if it's left in the centre the pressure is too low and if it's left at the edges it's too high.

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Nice tip, didn't know that one.

But there's a third rule - "If the chalk is completely gone, then its raining" ;)

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Nice tip that, wish I'd heard that earlier as my tyre fitter told me to stick to the same/manufacturers pressures when I fitted 235/85/16 BF Goidrich MT to my disco 1 and before I knew it both outer/inner edges had worn quite quickly compared to the centre. Have now upped the pressure and they seem to be wearing more evenly now.

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I thought I'd better update this thread, although it's an old one.

I wrote to General Tire to ask them the question:

"Please can you supply the design tire inflation pressure versus axle load for General Grabber AT2 tires. I use my vehicle both unladen, for general road use, and fully laden for expeditions that can include both on and off-road travel."

Their responses are below:

Initial response to my question:

We're sorry, but the vehicle manufacturer sets the inflation pressure specifications for their vehicles. While we, plus other tire manufacturers, may be requested to add input during the decision making process, ultimately the final decision is the manufacturer. This is because the vehicle's handling is set based on that pressure, and that means we are not in a position to offer contrary recommendations.

There is; however, an exception to this when a different tire size is applied. In that case, we match the load carrying capacity of the original tires as stated on the vehicle tire placard. In particular, the load table will not give you the correct information.

Unfortunately, we do not have a listing for your vehicle.

Would you please tell us what the vehicle tire placard says for the original tire size and inflation pressure?

I provided the following info from the Owner's Handbook:

Original tires were 750R16

Inflation pressures: Front: 28psi all load conditions

Rear: 32psi unladen, 48psi fully laden

Gross Vehicle weight: 6710lbs. (3050kg)

Maximum Front Axle weight: 2640lbs. (1200kg)

Maximum Rear Axle weight: 4070lbs. (1850kg)

EEC Kerb weight*: 4193lbs. (1906kg)

*EEC Kerb weight = Unladen weight + Full Fuel Tank (79.5Litres) + 75kg Driver

Their final response:

Thank you for the information. The load carrying capacity of a 7.50R16LT is: @ 28 psi: 1410#, @ 32 psi: 1530#, and @ 48 psi: 1996#

So the front GAWR appears OK, but the rear GAWR is more than the tire is capable of handling. This may mean the GAWR is listed incorrectly . Let's take the conservative approach and assume the GAWR is right - 2035# per tire.

In order to carry the same load, an LT265/75R16 needs to use: 22 psi, 25 psi, and 39 psi respectively (The last figure was adjusted for the rear GAWR.)

There is another general principle when changing tire sizes, and that is if the new tire size can carry the load at the pressures listed on the vehicle tire placard, matching the spring rate of the original tires becomes important - and since a tire's spring rate is very closely matched to inflation pressure, the principle says match the inflation pressure.

So our recommendation would be to use 28 psi on the front tires and 32 psi on the rear when empty and 48 psi when loaded.

One item of note: The rim width required for a 7.50R16LT is 5 1/2" to 7", while the LT265/75R16 has an allowable rim width range of 7" to 8". Please check to make sure your rims are at least 7" wide - as it is unlikely that the original rims were 7".

Customer Relations

Continental Tire the Americas, LLC

I don't know how applicable this info is to all you other non-OEM tyre users, but now you have it, for what it's worth. And I must admit, I've never linked vehicle spring rates with tyre pressures. I thought it interesting that the rear axle maximum loading exceeds the tyre capacity - according to General Tire :blink:!

I must admit, I've not yet tried the "chalk line" method, but will do so.

Mike

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I must say, they've done an awful lot of good work for you when the seem to know they're not going to get any money for it, thats customer service that is!

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I run my AT²s at 2.5 bar (36 psi) all around. At this pressure they feel stable in corners, handle rain well, and wear evenly.

.1 bar is enough to make it feel like a totally different vehicle (yes, I do drive my Rangie like a complete numpty)!

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The way I understand their reply, it's not the vehicles spring rate but the spring rate (flexibility) of the tyre that needs matching to the original hence their statement that it relates to tyre pressure. e.g. the higher the pressure the less "spring" in the tyre.

What's interesting is that they seem to say their tyres aren't capable of the rear axle loading yet all the suppliers sell them for just that!

Malcolm

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General Tire's load capacity data is wrong though. It isn't the same for all tyres of a specific size and you only gave them the original size of 7.50 - 16, not the load rating land rover recomend. As an example the current std factory fit 7.50 tyre is Michelin Lattitude Cross with a 112S load rating which is 1120kg per tyre x 2 =2240kg quite a bit more than the rear axle capacity. The std factory tyre for 130's and optional on 110's is The Michelin XZL which has a 116N load rating, which is 1250kg per tyre x 2 = 2500kg.

So what load index are they using for they're calculations? As they haven't stated.

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