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Tires/P to LT tire pressure

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I need your help!  Ive been obsessing over this for about 2 weeks now.  Vehicle in question is 2008 Toyota Land Cruiser (200 Series).  OE tire is a P285/60R18 114V.  TIP placard pressure is 33 PSI all around, even though GAWRs are 3595 front and 4300 rear.  Now, Toyota offered accessory wheels and tires through its TRD line.  17" forged wheels fitted with BFG All Terrain KO size LT285/70R17, Load Range E.  Tire specs being max PSI of 80, max load of 3195.  Toyota originally recommended 40 PSI for these tires on the vehicle, and there was an updated sticker with that tire size and that pressure to be placed on the doorjamb, reading 40 PSI.  Then shortly thereafter, they did a campaign to bring those vehicles in to reinitialize the TPMS (not simply reset), raise the pressure to 46 PSI and replace the doorjamb sticker with one saying new pressure of 46.  The P to LT tire pressure conversion has been explained to me to be basically this:  take the stock tire and pressure, go to load inflation table to see what load it corresponds to, subtract 10% from that load because its a P tire on an SUV, take that load and apply it the load inflation table for the new LT tire size, and get corresponding tire pressure.  When doing that, it comes to about 40, which Toyota initially recommended.  But why the change to 46?  Is there a minimum tire pressure that an LT E tire can be run at and is that the reason?  TRA lists pressures down to 35, but according to my research, the Japanese and European tire organizations (JATMA and ETRMO) list the minimum at about 58% of max pressure which would be 46 PSI for the Load range E.  Is that conversion factor of 10% or 1.1 or whatever applicable or not?  Maybe its only applicable when going to an LT tire on a light truck and not an SUV/MPV? Ive seen you mention that when going from P to LT, you add 15 PSI.  I guess thats when going to an LT tire in the same size?  LT tire in question is larger than OE tire and has more volume.  So many questions, I know!  What PSI would you recommend for that tire in my application?Looking for answers, thank you!

Answer
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.

*****************

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.  

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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.

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