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# Tires/P to LT tire pressure

Question

Stuart,

According to Tire Guides - a publication that lists the tire size and the inflation pressure, for all light vehicles sold in the US - a 2008 Toyota Land Cruiser came with P285/60R18 (like you said), inflated to 32 psi front/34 psi rear (which is different than what you said, but makes sense given the difference in GAWR's).  It does not list a 17" option.  That might be because it was an aftersale item - you did mention TRD!

If I do this math, the load carrying capacity of the tires at the above pressure is 2245#/2324# - which is 4490#/4650# for the axle - which is 108%/124% of the GAWR's.  Mmmmmm ..... very interesting.

So when I look up an LT285/70R17 I find that the max load is 3195# (like you said) at 65 psi (which is different!), but this was for a Load Range D.  My copy of the TRA yearbook says that LR E doesn't exist.

So I went to Tire Rack to see what they list for a BFG All Terrain KO - and they don't list the size, but there is one in the KO2 version, and its a LR E and listed as 319%# at 80 psi.   Mmmmm ...... Even more interesting!

And looking further in Tire Rack, I see that other tire manufacturers list LT285/70R17 LR E's at 3195# @ 80 psi.

So I do a bit more drilling and found a newer version of the TRA yearbook - and at some point in time, TRA amended their table and added a Load Range E in an LT285/70R17 - and the max load is listed as 3195@ 80 psi - BUT - all entries above 65 psi are 3195#!!  So the load carrying capacity maxes out at 65 psi (Long explanation why that is so!)

Doing the math, I get 39 psi/41 psi.

So why did they change from 40 psi to 46 psi?  I don't know.  It doesn't make sense to me either.  There must be some reason that doesn't have anything to do with the GAWR.

So what to use?  I would follow the latest version of the vehicle tire placard.  But keep in mind that this doesn't make sense relative to the GWAR's and if you find some reason that you want to use a lower psi, I feel comfortable with 39/41 psi.

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As I was reviewing the above before I pushed the reply button, I wondered if the difference in pressure might have something to do with the diameter.  NHTSA requires vehicle speedometers to be within 3% of the actual speed - and the nominal differences between those 2 tires is more than 3% - so I'm thinking the increased pressure has to do with that.

But my recommendation stays the same.  TRD would have done handling testing before changing the pressure listed on the placard - and I am not in a position to overrule them.

Tires

Volunteer

#### Barry Smith

##### Expertise

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.

##### 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.

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

Publications
You may want to visit my web site: http://www.barrystiretech.com/index.html

Education/Credentials
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.