Tyre Placards and Actual Pressures

Submitted: Tuesday, Jun 11, 2013 at 10:30
ThreadID: 102714 Views:2830 Replies:7 FollowUps:12
This Thread has been Archived
Howdy,

Sorry to be yet another tyre thread, but I am at loss to understand why the tyre placard in our recently purchased GU Patrol has 29 front and 32 rear (no mention about loads carried). Any ideas why?

This to me seems rather low, and backed up by the fact that when the vehicle went in for its 1000k service came out with 40 psi all round (with 48 in the spare....WTF?)

Just got back from a 2000k (mostly blacktop) trip up to Northern NSW and was running 38 all round (loaded to GVM). Tyres are still factory supplied D693's 275/65 R17.

Thanks all,

Lester.

Back Expand Un-Read 0 Moderator

Reply By: Ross M - Tuesday, Jun 11, 2013 at 13:28

Tuesday, Jun 11, 2013 at 13:28
Lester77
The tyre pressure on a placard is for the "ride quality" ( gosh these ride well, we will buy it concept) and not necessarily for the tyre load ability or life expectancy.
The inflated tyre on the spare is wisely inflated more than normal, so as it sits there unused it slowly goes flat over time and when you need it to use it most likely it has a chance of being high enough pressure to immediately use.

Yuo can let it down of course, if the pressure is too high, but you can't let it UP in pressure unless you also have a compressor or a very long air hose.
AnswerID: 512949

Follow Up By: Steve J9 - Tuesday, Jun 11, 2013 at 18:55

Tuesday, Jun 11, 2013 at 18:55
Patterns and construction will have a huge influence on not only ride quality but tyre wear in regards to pressure. A tyre (265/70R16 AT) with a load rating of 112 (pretty standard for most OE fitments) run at 36psi will work substantially harder than the same tyre with a 120 or above load rating at the same pressure. You are far better off having a higher load rated tyre and adjusting pressures to suit your circumstance.

While I have got you, I heard that someone in Vic is bringing in the PitBull range of off-road tyres, apparently the are great. Do you know much about them, they are made in the US
0
FollowupID: 791554

Reply By: Rangiephil - Tuesday, Jun 11, 2013 at 14:13

Tuesday, Jun 11, 2013 at 14:13
I think you are oversimplifying what car makers go through to determine tyre inflation recommendations.
The load rating of the tyres is always considered and the maximum load of the car. I believe ( am sure) it is illegal for a manufacturer to fit a tyre with a load rating and speed rating that is insufficient to carry the maximum load for the tyre at the recommended pressure.That is why the minimum load rating of the tyre is specified in the manual and the cops can defect you if the tyre load ratings are below that specified for the car.

The question of ride quality Vs longevity is another question, and I always wonder at the wisdom of people who deviate a lot from the manufacturers recommendation, as the manufacturer spend LOTS of money on testing to make the recommendations. The tyres are an integral part of the suspension and though you may save on tyre wear with high PSIs, maybe your shocks are failing quicker and the suspension bushes flogging out

Land Rover also recommend 28PSI for the front tyre and 38PSI loaded for the rear, and this is because the front weight of the car doesn't change much when you load it, in fact it may be lighter if you whack on a big van with a high towball weight.

I had a Suzuki Vitara once, and I couldn't figure why it has such an atrocious ride at 28PSI. All was answered when I looked at the tyre recommendations. It had 205x16s and the recommended PSI was 22PSI as the tyres had a much greater load rating than the loaded mass of the car. It seemed weird to run at 22PSI but that fixed the ride .
Regards Philip A

AnswerID: 512952

Follow Up By: splits - Tuesday, Jun 11, 2013 at 22:19

Tuesday, Jun 11, 2013 at 22:19
"as the manufacturer spend LOTS of money on testing to make the recommendations. The tyres are an integral part of the suspension "


Very true and they are designed in conjunction with the tyre company's engineers. If you change anything you will affect something else and it may not be immediately apparent.


"Land Rover also recommend 28PSI for the front tyre and 38PSI loaded for the rear, and this is because the front weight of the car doesn't change much when you load it "


My 03 Lux is much the same. 25 front at all times regardless of load and 25 to 34 rear depending on load for its 205R16 LT tyres. The much more lightly constructed 205R16C OME street tyres are also 25 front but 32 to 44 rear. The 255/70/15 LT are 28 front and 28 to 34 rear.

I have a set of LT and street tyres and swap them over depending on where I am going but I have have had the streets on the Oodnadatta Track and a few other roads in that area where they probably should not have been and they worked perfectly on factory pressures without changing them for different surfaces.

Toyota did not draw all of those pressures out of a hat, there is a good reason for them and there is a lot more to it than just ride.

0
FollowupID: 791579

Reply By: Bazooka - Tuesday, Jun 11, 2013 at 15:05

Tuesday, Jun 11, 2013 at 15:05
Sounds low given many passenger vehicles 2/3s its weight recommend around the 30 mark front and rear but as Phil says the vehicle manufacturer is the best starting point. 29/32 is probably an around town comfort rating - for better tyre wear and certainly for higher speeds you should add 3-4psi and more if laden. Don't forget placard numbers are COLD inflation pressures.

Here's what BS says on its website. Not sure if the max 40psi comment applies to heavy 4WDs and LT tyres. I doubt it.

-------------------------------
Tyre Pressure Tips

Contrary to popular belief, tyre pressure is not determined by the type of tyre or its size but upon your vehicle's load and driving application i.e. speed.

To find out what your car's tyre pressure should be, consult the manufacturers tyre placard usually found inside the driver's door sill, glove box, fuel filler cap or under the bonnet. The placard also displays the manufacturers recommended tyre sizes.

Tyre pressures should be checked when the tyre is 'cold', as pressure increases as the tyre becomes 'hot'. Take the "cold" reading and check them against the recommended tyre pressures from your placard.

Heavy loads or towing puts an extra strain on your tyres. So if your vehicle is fully loaded with passengers and luggage, the general rule is to add 28kpa (4PSI or 4lbs).

At high speed, (defined as driving at 120km/h for over one hour), your tyres will wear out twice as fast as when you drive at 70-80 km/h. If your tyres are under-inflated by twenty per cent tyre life can be reduced by thirty per cent. The rule here is to add 28Kpa (4PSI) from your Minimum Compliance Plate Pressure. Don't inflate your tyres above 40 psi or 280 kPa. When the tyres get hot from driving, the pressure will increase even more.

Believe it or not, checking your tyre pressure can have a big impact on our environment. An under-inflated tyre creates more rolling resistance and therefore more fuel consumption. By keeping your tyres inflated to their proper levels, you can help maximise your car's fuel economy and minimise its impact on our environment.

AnswerID: 512954

Follow Up By: The Traveller - Tuesday, Jun 11, 2013 at 15:35

Tuesday, Jun 11, 2013 at 15:35
Tyre pressures as stated on the tyre placard are a good starting point, but remember they apply to the original tyres. If, like many offroaders or tourers, you swap tyres then the placard pressures become less relevant. In NSW you are only allowed +15mm of diameter without an engineering certificate, but that allows a change from (potentially) a very "road biased" car tyre to a 15mm larger "off-road" light truck tyre, which is a very different beast.

The light truck tyres on my Hilux state (on the tyre) the maximum load carrying capacity with an inflation pressure of (from memory) 70psi. But Toyota, when they specify 28psi are assuming a lightly loaded vehicle with "car" tyres being driven on quality bitumen - not 800kg of fertiliser on dirt roads with A/T light truck tyres!

Then, of course, you alter the pressures to suit the road. Off-road work generally requires a reduction in pressure - if you have a pump to re-inflate them when you return to the bitumen! Travel over sand and corrugations is also often "eased" by marginally lowering the pressures.

Regarding fuel economy, generally speaking the higher the pressure the better the economy, but at the cost of reduced ride quality and grip (less tyre in contact with the road!). As mentioned in a previous post, reducing the compliance of the tyre also transfers more load to the suspension.

It used to be said that you should adjust your pressure to give a 10% (if I remember correctly) increase from cold to hot. So if you inflate to 30psi when the tyres are cold, after an hours driving they should read 33psi. If they read 36psi, then they needed more air to start with, and if they read 31psi they needed less. This will vary with load, so there is no single "best" pressure, even if you never leave the bitumen...
0
FollowupID: 791530

Follow Up By: The Traveller - Tuesday, Jun 11, 2013 at 15:41

Tuesday, Jun 11, 2013 at 15:41
I keep hitting the wrong "reply to" button, the post above was supposed to reply to the original poster!!

However to avoid another post, I also forgot to say that tyre pressure gauges can vary hugely. The best plan is to buy a good one and stick with it - relying on service station gauges to be accurate, or even "similar to each other" can result in problems.
0
FollowupID: 791531

Follow Up By: Member - John and Val - Tuesday, Jun 11, 2013 at 16:20

Tuesday, Jun 11, 2013 at 16:20
Errr... Traveler

I certainly agree that pressure gauges vary a great deal (I bought a quality digital one that was great, until it started doubling all readings!) I have found that the pencil type provide the most consistent readings, though absolute accuracy is pretty variable.

I can't agree with "Travel over sand and corrugations is also often "eased" by marginally lowering the pressures." In our loaded Troopy with fat tubeless A/T tyres I typically run 35/38 psi on bitumen, dropping to 25/28 on bad corrugations and 20/23 on sand, lower still when the sand is hot and fluffy. Obviously speed must be considerably reduced at these lower pressures. That's a fair bit more than "marginally"!

There's a very comprehensive blog here dealing with the relationship between tyre pressure and load bearing capacity. Well worth a read.

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

Lifetime Member
My Profile  My Blog  Send Message

0
FollowupID: 791532

Follow Up By: The Traveller - Tuesday, Jun 11, 2013 at 18:37

Tuesday, Jun 11, 2013 at 18:37
Indeed it is more more than "marginally"!

As you say, in soft sand and sometimes when rock crawling we can get down to very low pressures, but the risk of rolling a tyre off the rim is always present, and (also as you say) speed must be reduced to prevent the tyre from overheating etc. It also depends on the mass of the vehicle, the size and type of the tyres etc. as to what is required. I found that the cheesecutters on my old Hilux behaved very differently to the much wider tyres on, for example, my wifes Prado, presumably because of the far taller sidewall together with a lighter overall weight (unladen). I also have a suspicion that one of my old SWB "Series" Land Rovers has no tyre pressure at all, the age-hardened tyres stay up without air!

So my advice, when facing corrugations, is to reduce the pressure marginally - perhaps 10% or so - and see if it makes a difference. Especially the first time, when you don't know how that particular combination of tyre, vehicle and load will react. Then reduce a bit more if required. On the other hand, when experience tell you it is OK to drop straight to 16psi to travel over sand and the driver knows to slow down, then that's fine too!

The link you posted is indeed interesting, though it doesn't mention the issue of legalities. If the maximum tyre size on the vehicle was the starting size (stated as 265/75R16, but with mention of 17" alloys at some point), then changing to 285/75R16 wouldn't be legal in NSW without an engineering certificate.....though it would be OK in QLD and VIC......unless a suspension or body lift had been undertaken at which point it might not be. Don't you just love the rules!!
0
FollowupID: 791550

Follow Up By: Steve J9 - Tuesday, Jun 11, 2013 at 19:12

Tuesday, Jun 11, 2013 at 19:12
Patterns and construction will have a huge influence on not only ride quality but tyre wear in regards to pressure. A tyre (265/70R16 AT) with a load rating of 112 (pretty standard for most OE fitments) run at 36psi will work substantially harder than the same tyre with a 120 or above load rating at the same pressure. You are far better off having a higher load rated tyre and adjusting pressures to suit your circumstance.

While I have got you, I heard that someone in Vic is bringing in the PitBull range of off-road tyres, apparently the are great. Do you know much about them, they are made in the US.

This is my first post on this forum and I think I replied to the fisrt post instead of following the conversation, sorry
0
FollowupID: 791557

Follow Up By: Member - John and Val - Tuesday, Jun 11, 2013 at 19:27

Tuesday, Jun 11, 2013 at 19:27
Traveller,

Well worth reading the full account using the link given in the article. The detailed article doesn't deal with changing tyre sizes and the associated legalities. It discusses especially the impact on load capacity of reducing pressure, with specific detail for commonly used 4WD tyres. Scarey stuff.

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

Lifetime Member
My Profile  My Blog  Send Message

0
FollowupID: 791558

Follow Up By: Lex M - Tuesday, Jun 11, 2013 at 20:16

Tuesday, Jun 11, 2013 at 20:16
"then changing to 285/75R16 wouldn't be legal in NSW without an engineering certificate.....though it would be OK in QLD and VIC"

Not legal in Qld. +15mm increase maximum. And unless it's changed recently an engineer cannot approve it.
0
FollowupID: 791565

Follow Up By: The Traveller - Tuesday, Jun 11, 2013 at 21:41

Tuesday, Jun 11, 2013 at 21:41
I don't live in QLD, but I thought they had approved the new VSB14 schedule that specifies that 4WD's (specifically) may have +50mm tyres fitted without engineering, with the caveat that the overall lift cannot be more than 50mm (i.e. 50mm suspension lift cannot be combined with any tyre size increase).

NSW have not approved VSB14, but they are apparently using it as a "guideline" for engineering inspections.
0
FollowupID: 791572

Follow Up By: The Traveller - Tuesday, Jun 11, 2013 at 22:10

Tuesday, Jun 11, 2013 at 22:10
"Traveller,

Well worth reading the full account using the link given in the article."

Indeed, and I did read it. It is very informative about load ratings, speed and temperature. I was merely commenting that it doesn't mention legalities, so anyone inferring a recommendation could run in to problems other than delaminated tyres!

Like many aspects of 4WD modification, tyres are a minefield of technical solutions and legalities where what work best may not be legal in all, if any, states. Load rating, pressure and speed are important, but so are legalities, impact on handling, GVM, unsprung weight, fuel economy etc. etc. And everything has trade offs against everything else!
0
FollowupID: 791576

Reply By: Rockape - Tuesday, Jun 11, 2013 at 20:15

Tuesday, Jun 11, 2013 at 20:15
Lester,
This is what I ran my old Troopie at. I don't go by placard due to the fact the tyres are not what was fitted by the manufacturer. All the pressures I use are based on the rating of the tyre. You will note the steer tyres are only at 28 psi but that is fine for the front axle loading. Here is a set of calculations based on the vehicles load.


? Front axle 1260 Kg.
? Rear axle 1760 Kg.
? Total GVM 3060 Kg.

Tyre pressure.

? Front tyres 28 PSI.
? Rear tyres 38 PSI.


Formula for pressure.

? Load rating of 285/75/16 tyres 1500 kg at 65 psi.
? Rears are 880 kg per tyre which is 59% of max load carrying capacity.
? Fronts are 630 kg which is 42% of max load carrying capacity.

By the way my spare is always pumped up over 50 psi. I can let it down easily, but it is harder to pump it up.
AnswerID: 512964

Reply By: mikehzz - Wednesday, Jun 12, 2013 at 07:57

Wednesday, Jun 12, 2013 at 07:57
Most tyre places put too much air in the tyres in my opinion. I normally run the factory placard pressures, have done for years. I get 80-100k out of a set of tyres plus a better ride and my shocks last forever. You can feel the car disintegrating around you at high pressure, no rattles in my cars. :-) I do high kms on all sorts of crap roads too.
AnswerID: 512982

Reply By: Member - Lester77 - Wednesday, Jun 12, 2013 at 09:58

Wednesday, Jun 12, 2013 at 09:58
Thanks again all. Off to drop em back to what Mr Nissan says (+2). Still at a loss to understand why the dealer put so much over and above the placard.

Makes perfect sense about the spare now too.....D'oh.

AnswerID: 512984

Follow Up By: splits - Wednesday, Jun 12, 2013 at 10:34

Wednesday, Jun 12, 2013 at 10:34
"Off to drop em back to what Mr Nissan says (+2).

That reminds me of the time I was working for a small company in the 1970s. The boss bought a new Jaguar and the handbook said "For sustained speeds above 120 MILES per hour, increase tyre pressues by 2 psi".

An extra 2 in your Nissan should have it going like a rocket!


"Still at a loss to understand why the dealer put so much over and above the placard."

I think the main problem is there are too many different cars and tyre sizes with different pressures for anyone to remember and they don't have the time to look them up. You also have many cars with tyres that don't match the placard.

I have three stock sizes on my 03 Lux all with different pressures. My '91 model has five in the handbook. All have different pressures including two of the same size but with different load ratings.

0
FollowupID: 791607

Reply By: Rangiephil - Thursday, Jun 13, 2013 at 08:42

Thursday, Jun 13, 2013 at 08:42
I think all apprentices at dealers and tyre dealers have a mantra drilled into them at TAFE or somewhere.
"30PSI good 40PSI better". maybe they have to read Animal Farm.

Or maybe the boss tells them to do it so that it is demonstrable to the client that they have actually checked the tyres, even though said client has to then waste 5 minutes deflating tyres to correct level.

Just thought of third possibility. they put in a lot of pressure to seat the bead , then save time by not letting the tyre down to the correct level.

All I know that whenever I get new tyres or have a balance , my tyres always end up with 40PSI!!!!!!
Regards Philip A
AnswerID: 513031

Sponsored Links

Popular Products (13)