Tires: LT vs P for 265-75/16, passenger car tires, car tire sizes

Question
Thought so.  But then how can the factory 285-75/16 LT's be placarded for only 35 psi?  Seems to be a large disparity.
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Followup To

Question -
My Hummer dealer replaced the factory P tires with the same size LT load D tire, and said the tire pressure instead of 30 psi per placard should be 37 psi.  From what I've read, I think 45 psi is more like it.  Who's right?

Answer -
Dave,

This is why I really dislike Car dealers getting involved with tires.  You are correct.

Answer
Dave,

In the days when passenger car tires were used on passenger cars and truck tires were used on trucks, there was a clear division in the tires - basically by size, but there was also a difference in the way these types of tires (and their service) was analyzed to try to understand the theory behind what worked and what didn't.  Remember, all this work would have been done prior to computers and all a guy had was a pencil and paper and his brain.  Complicated analyses were just too time consuming.

So truck tires and passenger car tires developed along different lines and developed their own way to describing the relationship between inflation pressure and load - called the load curve and commonly expressed in the form of a load table.  If you know anything about tires, you'll know that the larger the tire, the more load a tire can carry.

When smaller trucks were developed, vehicle manufacturers decided to use large passenger car tires, which have a softer ride.  So new larger passenger car tire sizes were developed based on the previous formula, and this resulted in an overlap in the sizes.  To distinguish between the 2 different types, the letters "P" and "LT" were put in front of the size.

But the interesting thing about the 2 different formulas was that P tires would carry more load than LT tires for a given size and pressure.  Rather than rewrite the whole formula - after all these formulae had been successfully used for decades - they decided to limit the lower end of the LT sizing tables and the upper end of the passenger car sizing tables.  Unfortunately it took over 10 years between the discovery of the problem and the limitations being agreed upon, and many tire sizes now exist in the overlap.

One way to look at this is that it isn't the size of the box that determines how heavy on object the box can hold - it's how much cardboard is used that matters.  

With these 2 types of tires, the standard has been set and it is up to the tire manufacturers to develop a product that works for those conditions. As a result, these products are quite different internally and the constructions take advantage of the difference - the LT tires don't flex as much so they can be built stronger, while the P tires are built with flexibility in mind.

BTW, P tires are derated 10% when used in LT applications - Pickup trucks, Vans and SUV's.  So what you see printed on the sidewall is not necessarily the actual load capacity.

Hope this helps.