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Tires/Tire Inflation Calculations - Weight & Pressure


In a 11/18/2009 response to an question entitled "Tire Inflation on BFG's" you stated the following:

Let's do the math first.

According to Tire Guides, a 2002 Land Rover Discovery Series II came with 2 different tire sizes, and one of them was 255/65R16 105H inflated to 28 psi front / 46 psi rear.  ...  That means the load carrying capacity of the tires is:  825 kg (1819#) front / 1030 kg (2271#) rear.

It does look like you did the math, but can you share what those formulas/calculations were that you used to get the load carrying capacity of the stock tires at the vehicle prescribed tire placard pressure?  I would like to apply them to other situations.  Also, how did you calculate the pressure needed in the new/different tire sizes using this data?  



The load calculation is pretty complex.  In order to make it simple, the tire standardizing organizations publish tables that have the values in them.  

Unfortunately, those tables are covered by copyright laws and are not allowed to be published.  They publish them in books that are pretty pricy.  (It is, after all, that organization's only source of revenue.)

The good news is that you can find portions of the tables on the internet - especially through tire manufacturer's web sites.

Oh, and a caution:  There's a reason why tire manufacturers refer to the vehicle tire placard for inflation pressure.  It's because there needs to be a certain amount of unused load carrying capacity - good engineering practice.  The vehicle manufacturer will have a handle on how much load transfer takes place in its vehicle and adjusted the inflation pressure accordingly.  The tables only provide minimums without any consideration of the dynamics of the vehicle.

Personally, I would be very careful using the load tables - especially if a vehicle manufacturer has already provided an inflation pressure specification.


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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

You may want to visit my web site:

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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