During conversations with others, some travellers are not aware of correct tyre pressure when travelling on the tar whilst towing.

If pressure is too low tyres are said to be in the ground creating unnecessary drag and being concave, if pressure is too high tyre is convex wearing out in the centre, although the tyre is running 'on top of the ground'.

Manufacturer may have 30-35psi recommended pressure but this is too low for a 2 tonne van, the tyres are running into the ground, have the correct pressure means lower revs for the same speed plus better economy.

I have found 2 tonne 40psi and 3 tonne 45psi makes an incredible difference in terms of economy, with no convex or concave with the tyre, realize most are aware but not all, as discussion have shown, have a good one.

Gday...

I haven't heard of it called "running into" or "running on top" bitumen before - but can understand the description.

If they are at the correct pressure does that mean "running within" the bitumen

However, I would be somewhat disturbed if a manufacturer of a 2-tonne van stipulated (recommended?) 30-35PSI for that van's tyre pressure.

I would suggest that a van weighing between 2,500Kg-3,000Kg would (should?) have Light Truck tyres fitted and they be inflated to about 50PSI ... perhaps 55PSI. I would think 45PSI for a 2-tonne would be closer to correct.

It was recommended that my van's tyres should be run at 55PSI, but I kept my eye on them when I first hit the road, and found that 50PSI is where they show the best wear characteristic. I got 65,000Km from my original set, and so far I have got 32,000Km out of their replacement.

Cheers - and happy travelling "within" the bitumen - John

Bruce and Bev wrote:

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Hi John. Ive got new light truck tyres on my DMax. The manufacturer recommends 40psi. The retailer told me to put 45 in rear and 40 in front but only when towing

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Gday...

I formed the opinion the OP was talking about tyre pressures for the van  hence my waffle 

However, if we are talking about vehicle pressures - a different kettle of fish

Completely dependent on the vehicle, the type of tyres fitted, the brand of those tyres and the the conditions that the vehicle is being used in - normal/highway, towing, off-road etc.

With vehicles, there is a tyre pressure plaque (usually inside the driver's door) which states the vehicle manufacture's 'recommended' pressures.

If you visit the tyre manufacturer you will find they have different "recommended" pressures - usually higher pressures than the vehicle manufacturer. 

This is because the vehicle manufacturer wants your ride to be 'comfortable' hence lower recommendation - and the tyre manufacturer wants the handling (and probably tyre wear) better.

I am sure most of us decide from experience the 'optimum' pressure their tyres need to run at to give good handling, good wear and an acceptably comfortable ride. These 'optimum' pressures will vary should a change of brand of tyre occur as tyres wear out.

The manufacturer of my vehicle states (on that little plaque) 32PSI front/34PSI rear for 'normal' driving ... and 34PSI front/54PSI rear when heavy load/towing.

I run 34PSI all round when not towing and 36PSI front/55PSI rear when towing.

I arrived at these pressures from observing wear pattern/handling/road noise/longevity from the last three sets of the same Cooper tyres I have used.

When I go off-road, without the van, I drop to 28PSI all round ... in rocky country country 24PSI ... sand down to 15-18PSI. Tyre pressures MUST be returned to higher pressures when returning to 'normal' conditions.

The pressures you have quoted for your DMax may certainly be the 'optimum' pressures - I have no idea. Depends on what the vehicle manufacturer recommends, what the tyre manufacturer recommends, what other driver's recommend and how you observe the wear/performance etc of the tyres at those pressures.

And if you replace these tyres for a different brand - you may have completely different pressures again.

Cheers - and happy motoring - John

tezza wrote:

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i have found a rule of thumb if tyres are at correct pressure there is less than 5psi difference between cold and hot. if there is more difference than that it means tyres are flexing too much which generates heat.

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Agree with Tezza 3-5 psi increase (3 for small cars and typically 5 for say 265x75 X 16 ie our Toyota).

We had tyre dogs on truck rear and van dual axles. Pressure within 5 psi range generally but went up to +8 psi at 42 degree day so OK.

THE MAIN POINT WITH TRYE PRESSURE IS WHAT WEIGHT (THE LOAD) IS ON EACH TYRE.

If I said we run 40psi front and 60psi rear on our Toyo Landcruiser 79 series ute some would be horrified,,,, but the pressures are directly related to the load (1.4 tonne on front and up to 2.3 tonnes on rear axle - with increased GVM to 3.7 tonnes fully approved) and tyres wear has been even and symmetrical for the last 9 years.

Van with dual axle has 750kg on each wheel and 40psi is perfect for the van. When down to 35 psi the first stage of sideways swing occurred (ie tyre walls flexing), so at 40psi I know I'm close to ideal pressure, which when travelling goes up to 43-44 psi on longer runs.

Yes I practise 4 psi rule and it did take sometime to balance it out. I ran about 1200 kilometres over 2 days testing at every meal break and over night stops.

Now fairly comfortable with my pressures. I run as recommended by the ute manufacturer 36 front the back up 2 instead of 36 now 38. The caravan was a guess at first as I was following a rule I was recommended same as the drive but tyres heat was high now run 42 in a single axle caravan weighing approximately sub 2000 kgs after 2 hours very little expansion max. 4 pounds. Thats 1800 kgs on the axle.

Very happy with that but did take the time and the Cheese and Kisses was getting a little trouchy when we would pull up and I paid more attention to the tyres then her instead of making coffee.

Hope this helps. Ralph