Tyre Placards and Actual Pressures

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.

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

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.

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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.

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.

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.

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.

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