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Thread: 3 PSI rule, does it apply to dirt roads

  1. #1
    Join Date
    Feb 2005
    Bracken Ridge - Brisbane - QLD

    3 PSI rule, does it apply to dirt roads

    I had to plug a tyre on my recent trip which meant I closely monitored it tyre pressure.

    I was running 22psi prior to the puncture, I adjust all tyres to 30 as we had a combination of dirt and bitumen for the day.

    On dirt each time I checked all tyres were around 35psi.......I assume the 3psi theory would mean my pressures were to low??

    I come to the conclusion that the 3psi doesnít apply to dirt and lowered my pressures to 25psi.

    Iíve never done the 3psi test on bitumen......40 all round and leave it at that.

    2000 110 Hardtop

  2. #2
    Join Date
    Feb 2011
    Hobart, Tasmania, Australia
    I was told the 2psi/4psi rule - i.e. 2psi difference for road vehicles and 4psi difference for 4x4.

    Tried it for a while and couldn't ever find the Goldilocks pressure. Seemed to always vary too much depending on the weather and specific road.

    So now my laziness is to just use 40psi for road, 30psi for dirt and 20psi for sand, as my starting point and then adjust from there if necessary. I've found with my setup on 285/85R16 on Dynamic 16" x 8" wheels that 20psi off road results in the bead seal failing too frequently and that sand will often still requires lower than 20psi.

  3. #3
    Eevo's Avatar
    Eevo is offline OldBushie Gold Subscriber
    Join Date
    Sep 2012
    Adelaide Hills
    when it comes to tyre pressure, everyone is an expert and has a different opinion.

    i dont think the 2/3/4 psi rule works. there are too many other ambient factors.

    i do use 40psi for road, 30psi for dirt and 20psi as a a guide.
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  4. #4
    Join Date
    Dec 2008
    3 PSI is neither here nor there in the world of both privately owned and service station tyre pressure gauges. They can vary among themselves by reading up to a 15 PSI difference for any one individual tyre. For the best accuracy I can manage I eschew the cheapies and have a VDO gauge which is never dropped or abused.

  5. #5
    Join Date
    Feb 2005
    Bracken Ridge - Brisbane - QLD
    Yeah always wondered......but never tried the theory.

    Anybody use the theory??

    2000 110 Hardtop

  6. #6
    Join Date
    Mar 2007
    Warwick Qld
    If I put on a new set of tyres, I'll watch the pressures for a while. I try to do it at around 25* ambient temp. If the pressure climbs more than 4 psi in about 30 minutes, I'll add air in about 5 psi increments until it settles to about 4-5 psi difference. That is what I usually run from then on (with the same load, etc), minus 10-15 for dirt/gravel and sand starting about 20. Of course, tyre construction plays a role as well. Because of variations in road surfaces, corrugations etc they are only baseline figures and need to be adjusted accordingly.
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  7. #7
    Join Date
    Apr 2011
    I find the rule works in theory, but can be difficult to put into practice sometimes. Tyres facing the morning sun can rise 3-4 psi in a very short time. I generally run 30/36 f/r.... but I drive a mix of dirt and tar roads daily. Will lower if doin continual dirt or raise for tar only, and raise again for load if needed. I found too higher pressures on dirt caused rocks to drill through the cases and caused less traction. I use 4 psi, gets me close.
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  8. #8
    Join Date
    Jan 1970
    In my D2a Ripping It Up
    For the driving i do i have two pressure`s to go to and i don`t stress about the 3 psi rule or any other pressure difference theory
    40psi + ON ROAD
    25psi For Dirt and Off Road unless it is for a few hours and high speed on dirt roads then a bit higher
    On sand it depends on the time of the year i have found so can vary in pressure`s quite a bit so for me no magic number just start at about 22 psi and lower if needed
    cheers chris

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  9. #9
    DeanoH is offline ChatterBox Silver Subscriber
    Join Date
    May 2009
    West Gippsland - Victoria

    'More of a guideline than a ruls'

    ...the code is more what you call guidelines than actual rules - YouTube

    There's too many variables to have a hard and fast rule regarding tyre pressures, load, tyre construction, speed, sunlight/ambient temp to name some. Ricks observation re sun on the tyre is right on the money eg. traveling north up Knolls Tk in the Simpson Desert I'd decided set the D2's front tyre pressures to 18 psi (cold) but lazy me ended up with 17 psi front left and 18 psi front right, she'll be right I thought. Over the next hour or so the steering got worse and worse till I had the steering wheel at 45 degrees and was literally 'ploughing' my way forward. I thought I'd broken something but it was the hot morning sun on the RHS of the vehicle which was already 1 psi 'high' and gained another 4 psi with the suns heat giving me a 5 degree or nearly 30% differential in front tyre pressure. Equalising the tyre pressures solved the problem.

    My general 'rule of thumb' is down (from manufacturers specs.) 25% on good dirt, 33% on poor dirt and 50%+ on soft sand but this varies with the vehicle/conditions.

    With the Oka (say 5 tonne loaded) and running 325/85 R16 Michelin XML's (very aggressive 38"s), I run them at 60 psi on the bitumen hwy (this tends to minimise lug wear) at 95-105 kph, 35-45 psi on dirt hwy ie Birdsville Tk at 80-90 kph (takes a hell of a lot longer to stop on dirt), 25-35 psi on poor dirt and in sand a lot less. ie Ruby Gap river bed 15 psi at 'bugger all' kph

    So basically 'horses for courses'. Just beware of the dick head knuckle draggers at you local tyre service who will often tell you to run your tyres at stupidly high pressures, absolutely NFI, IMO best to use the tyre placard as your starting point .

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