Tyre pressure questions

I generally have:
38 on road
30 on gravel
24 on Qld beaches
16-24 on Fraser & Moreton inland tracks
Lowest I have gone to is 12

4psi rule = the differnce in pressure between cold and hot tyres should be 4 psi. if the difference is more you :
1 are driving too fast
2 need more air in your tyres

The answers depend on the tyres, the load and the speed.



Cheers,
Peter
OKA196 Motorhome

Every car is different. There is no one size fits all. People try tell me to run 32-34 psi on gravel, but yet i run 30-32 psi(tyre placard recommended pressures) all round on bitumen so how does that work. Its pretty much trial and error to get things right, drop your pressures to what suits your driving style, your speed, your load, the terrain, tyre type, etc etc.

And i believe with L/T 4wd tyres the difference in pressure should be more like 6 psi, 4psi is for passenger construction tyres.

Hi JohnnyT

I know any tyre comments are subjective, but for what its worth, here is my Pirelli experience.

I bought a s/h Landcruiser 100series about 3 years ago. It had a very well worn set of the previous model Pirelli Scorpion ATR's on, and I was very happy with them - quiet and good roadholding in all conditions on bitumen, good on the dirt and on the soft sand. We did the Simpson and the rocky surrounding roads on barely legal treads with no punctures or blowouts.

12 months ago, I replaced the Pirelli's with a new set (with the current tread pattern) in preparation for a big Kimberley/Pilbara trip. The new Pirelli ATR's were just as good as the old set on bitumen, nearly as good on a dirt road, but do not grip well at all in soft sand, and I bogged many times in situations where I do not think I would have bogged with the old set.

I use 40psi on bitumen, 28 on dirt/stones/gravel and @18 on soft sand.

Hope this helps with your decision.....

Cheers Dalb

Hi John.
I read somewhere once the following formula:
Road tyre psi = (max allowed pressure x actual load) divided by maximum load then add 4psi.
The max pressure and max load are on the tyre.
Example:
max load single axle 1360 kg, max pressure 65 psi.
vehicle loaded weight 2500 so 625 each wheel.
(65 X 625) divided by 1360 =30 add 4 = 34 psi.

you can then reduce pressure by 20% for rough roads with a max speed of 80 kph.

regards Cruiser

Hi Johnny, The "rule" you refer to is the 4 PSI rule & is applicable to bitumen road use only.

Inflate your tyres to the recommended tyre pressure on you vehicle tyre placard when they are cold. Take your vehicle for a half hour spin to warm the tyres up & then re check your pressures. In an ideal situation they should now be about 4 PSI above the cold pressure reading. If they are higher than 4 PSI above the cold pressure then add a little more pressure.

If they are less than 4 PSI above the cold pressure when warmed up then your cold pressure is a little bit high.
The above info comes from a booklet put out by Cooper tyres, " 4WD Drivers Guide" & you should be able to pick up a free copy at most tyre outlets. Cheers

Hi Johnny,

As other replies have said the amount you lower your tyre pressures will depend on a lot of factors incluing the manufacturers recommended pressure and the load in your vehicle.

As a general rule I would drop by tyre pressures by 10% for good Gravel roads, and 20% for bush tracks. Remember tho you must also lower you speed if you lower your tyre pressures.

I've put a couple of blogs up on the blog section regarding tyre pressures and tyre deflation and they are a good starting point. They are not the be all and end all as everyones situation is different.

Have a read of the Tyre Deflation Blog Here.

And the Tyre Deflator Blog here.

Remember also that you need a good compressor to re-inflate your tyres afterwards.

If your not a member of a 4x4 club and your new to 4 x 4 driving you may find that joining a club will make a lot of information and knowledge available to you. Most clubs are only too happy to give you some advice and tips and pointers, and many have training/awareness days for new members.

This is from my personal experience over many (too many) years,
If you have good suspension and a reasonably tough vehicle then as long as you can cope with occasional discomfort it is best to run you tyres at close to maximum recommended pressure (in most cases just under 50psi). Personally, I couldn't care less about the 4 psi rule.
You may benefit from dropping the pressure for sand driving but never for the forest sand tracks such as inland tracks on Fraser Island.
If you get bogged (never had the experience apart from "staged bogging" at the 4wd training course) you will need to drop the pressure but the best option would be to avoid soft sand/mud in a first place.
Obviously this is quite contrary to the generally held beliefs perpetuated by tyre manufacturers and dealers, you could almost think that they have vested interest :), so you may expect that majority opinion will be completely different.

Cheers