My NOT so good 4WD video clip

whats the 4psi rule?
I had the pressure down to about 32. However those gauges are not very reliable. Went to pump it up before heading home and it showed 38psi at the servo ??!!
JS.

The 4lb rule is check your tyres cold as in first thing in the morning. Check them again half an hour down the road. They should be 4lb higher

If not pump them up a bit more Under inflated tyres heat up more

When towing I work on 6lb and they have stayed good for 60,000k so far.

You have learned that no two gauges are the same.

If you use the above rule what the gauge says doesnt really matter as its all relative to loadings temp etc.

I have a TPMS and two gauges Only 1 agrees with the TPMS so I use it.

I'd agree 100% with Swaggie, though the actual pressures you use depend on a lot of factors, especially type of tyres and loading. With fat tyres I use 30-35 psi (when the tyres are cold) on bitumen for our Troopy, and I'd drop to about 22-25 as Swaggie suggests in the sort of conditions in your movie, maybe down to about 18 if necessary. I usually think in terms of % of bitumen pressure - I go down at least 25% on rough gravel, 50% for very loose or sandy. As well as more comfort for vehicle and occupants, the aim is to increase traction through increased footprint - half pressure will roughly double footprint.

Last resort for me is about 10-12 psi. Below that there is real risk of the vehicle actually walking out of its own front tyres. At such low pressure you must drive slowly and steer very gently.

The 4 psi rule - basicly, run your tyres at a pressure where they flex enough to warm up so that the pressure rises by about 4psi. It's a very rough guide, since there are so many variables - hot bitumen, sun shining on tyres, etc , but useful to get a feel for what's about right.

It's important to drop your speed when running at reduced pressures. Certainly never attempt highway speed without highway pressure!

Agree completely with your comment about the accuracy of gauges. Recommend don't use an ultra cheap one, but use the same one consistently as your reference. Every gauge is different, but consistency is more important than absolute accuracy.

Cheers

John

PradoMad, It is quite possible that there is nothing wrong with your tyre gauge. When you set your pressure at 32psi were your tyres fairly cool? Then on arrival at the servo you probably had been driving at reasonable speed for a time and your tyre pressure had increased to the 38psi you observed due to heat build-up.

Next time at the servo compare your gauge indication with what the servo gauge shows. You may find only 1 or 2psi difference and that is to be expected. If there is more than 1 or 2psi compare your gauge readings with some other gauge.

Personally I believe the 4psi rule simply wouldn't cut it in this situation and nor would I even consider using it. Judging from the loose service, and that I would be in low range with a peak speed of around 50kph (not in this case mind you) I would simply drop pressures to around 25psi and then pump up again before exceeding 80kph....but in any event prior to getting back on quality gravel or bitumen. The 4psi rule is not just proportional to flex in the side walls, although this will increase temperature and pressure, the greatest contributor to exceeding the 4 (some also say 6) psi rise is speed. So, if you are dropping pressures it is always advisable to drop speed also.

...as pointed out by John above

Great advise ALLAN B
thank you
JS.

excellent advise, thanks
JS.

I thought that the Prado was auto, therefore this technique would not work? Maybe my ears are failing me haha

very observant!
Video was shot in BRIGHT VIC. Awesome country!!
JS.