<span class="highlight">tyre</span> <span class="highlight">pressures</span>

hmmmm.............I trust you carry a compressor and not all these surface types are on the one trip - I also gather you are from the east so here goes...........fire trails 25-30 rocky terrain 25-30 corugated tracks 25-30 (see a pattern here?) mud/clay 25-30 sand 15-20 rgds

G'day, I sent this reply to Angler a few days ago. Before you start dropping pressures, it is important to get the normal road pressure right first.

Here's my 3 cents worth. I just bought a 2004 GU III Patrol 3.0TD manual, and had the same questions. 27,000k's and the rear tyres are nearly smooth in the centre. I have Bridgestone Dueller A/T D693's in 265/70-16 fitted.

I started by ringing the local Bridgestone dealer and asked the question, then the local Bob franchise, and surprise, surprise, surprise, got two different answers. Maximum pressure was anywhere between 50 and 65 psi.

Time to get technical. An email to Bridgestone Australia was quickly replied too. Maximum tyre pressure - 36psi !!! So from there I went and got the axles weighed (full fuel tanks and girlfriend, and myself in the vehicle, and it weighed 1,320kilo's - front, 1,240 rear) and from there it was simple math.

The equasion went like this (front) - weight (!,320) divided be 2=660. Maximum tyre load for the D693 is 1,120 kilo's@36psi, 36 divided by 1,120=0.032143. So 660 multiplied by 0.032143=21.2psi. Now I added another 5psi for high speed running (110kph) and with 26 psi (cold) in the front it steers nicely, doesn't roll about overly, and give a smooth and comfortable ride. The same equasion was done for the rear and I run 24psi (cold) there. This is the best starting point.

From here I will monitor heat build up - I am aiming for a 4psi pressure build up after an hour of highway running. If I get more than a 4 psi build up, I will increase the cold pressures by 2psi each time until I do. Conversely if I get less than 4psi, I will start dropping pressures.

Obviously loading the vehicle will change all these figures, but it does give a good starting point. Hope this helps. Cheers.

G'day MrBitchi (QLD). Thanks for your follow up. Just to reinforce my logic. I emailed Bridgestone Australia about load/maximum pressures, and they replied with the maximum load per tyre on the Dueller A/T is 1,120 kilos per tyre at a maximum pressure of 36psi.

Other tyre brands may have the same load rating, but at higher pressures, depending on construction/ply rating etc, therfore each individual vehicle will be different for each set of circumstances, load/speed/conditions etc.

Example, if my axle weight were to be 1,450kilo's front and 1,550 rear I would increase my starting pressures to 28psi front and the rears to 30, then apply the 4psi heat build up rule.

I accept that in hot climates heat build up will be greater, specially during the heat of the day, but this formula is designed to give a starting point only and each individual driver should do their own calculations to suit their operating conditions.

I hope this clears up any confusion.

Cheers :-)

Way too many variables.
how heavy is your truck?
Are you towing?
what size tires?
what size rims?
what weather?
How fast?
what sort of terrain in each -
eg loose, hard packed, mixtures, how muddy how corrigated, how deep corrigations? steep slippery, flat, downhill, puddles, bogholes?

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fire trails 10-38
rocky terrain 10-38
corrugated dirt roads 10-38
muddy/clay slipperry stuff 10-38
sand etc 10-38