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Tires/Tyre pressure chnage to LT


Hi Barry
I drive a Mitsubishi Triton / l200 twinn cab 4wd ute which came from factory with 245/65 R17 RF HT bridstone 840 tyres with a load index of 111
The factory plate advises 29psi front and rear up to 3 passengers, and then 29 / 42 when towing or at max GVMthe vehicle curb weight is around 1.8 metric tonnes and Max GVM is around 2.8 tonnes.

I have switched to a more off road bias tyre tread pattern is a between a AT and road terrain type, and raised the profile only to gain a little under the axles...  new tyre is LT245/70 R17 with a load index of 119/116R. The tyre dealer has put 40 psi in the tyre and advise not to go below 36 on the highway..  The tyre manufacture has advised F/R 31 psi road and for MAX GVM F/R and 34 / 46 Laden. So I'm asking for some independent advise that would give me a balance between good tyre life but good grip. I suspect the 31 psi F/R is a little low for tyre life. I currently have them set at 34 front and rear,  I tend to drive highway, but my local rds are gravel/clay based. driving on these these has the back feeling loose
I feel the right Front tyre pressure for me to stop the sides of the front tyres knocking my luggs of while cornering and highway fuel economy and tyre life, at F32 and set the rears at R 32. which gives me  load capacity to take the family for a short drive do the run around local, and have the rear gripping in the loose stuff.

Your thought would be great.


Doing the math, you need to add 11 psi to all the pressures listed by the vehicle manufacturers to get the same load carrying capacities.  Interestingly, your tire dealer wasn't too far off the mark.  (They usually aren't that good!#  I also find it interesting that the "tire manufacturer" got the wrong answer.  I wonder if an engineer did the calculations and was aware of the side notes listed below.

Some side notes:  P type tires have to be derated 10% when used on light truck type vehicles #SUV, pickups, vans, trailer, etc#

In order to get the same load carrying capacity, an LT tire has to use 15 psi more.

And I hope you now realize why I am not a fan of making the switch from P type to LT type #or vice versa).  The pressures are different and the vehicle was set up for the one set and not the other.

And one last thought:  Tire pressures affect all kinds of things - BUT - the single most important thing is that it affects durability.  Low pressures are the single most common reason why tires fail - and sometimes that has tragic results.  Don't be part of that statistic.


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Barry Smith


I have over 30 years experience in the design, manufacturing, and testing of tires. I have served as the technical advisor to the "800" number. I have authored or co-authored many publications - usually without credit. I can answer almost any technical question, but please don`t ask me to compare brands. I probably have prejudices because of my work experience.


Member SAE (Society of Automotive Engineers) Member Tire Society (Tire Technical Organization) SCCA Regional Competiton License holder Authored many training manuals on tires, their care and use.

SAE (Society of Automotive Engineers) SCCA (Sports Car Club of America) Tire Society

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I am a graduate of the University of Dayton, Dayton, OH - BME, 1971 I attended graduate school at the University of Akron - but did not complete my PhD - something I regret.

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