You would have to post up exactly which tires you are referring to. The reason for the higher pressure is to get away from sidewall flex when loaded, as flexing develops heat, which kills tires. Load range E tires have higher load capacity that any load range below it. Unless you are planning on towing heavy with a half ton, stick to C or D tires if you can, as you sacrifice ride quality with E tires without weight.

 

im putting the ko2's on my truck. based on bfg's load tables and pressure calculations, it would want far more than 40 psi for my application. in stock form, the truck calls for 35 psi for the p-rated tires that came on it.

its more of a general question. why does a larger, stiffer, wider tire need more psi to hold the same weight?

 

You would need more than 40psi when using for towing but for a daily driver 40-45psi will likely be more than ok to run.

I'm running 285/75r17s E loads with 42psi and get a good contact patch.

 

You would need more than 40psi when using for towing but for a daily driver 40-45psi will likely be more than ok to run.

I'm running 285/75r17s E loads with 42psi and get a good contact patch.

agreed, and this is what I run with my 295 duratracs, but I am interested in why do larger, stiffer tires need more psi to support the same weight as a small p-rated tire

stock p-rated-----P265x65r18 ---35 psi
ko2 E-rated-------33x12.5 r18---40+ psi (much wider, stiffer side walls --->needs more psi??)

im just looking for a good explanation as to why this is the case

 

As I said, that thicker, heavier sidewall does not like flexing, as it will build up more heat. Here are more layers in the side wall, so more friction when it is flexing. Add air pressure and the flexing is lessened, so less heat. He P metrics have very little material in the sidewall, so less heat build up from flexing.

 

im just looking for a good explanation as to why this is the case

Imagine blowing up your typical party balloon with air pressure you generate. Not too hard?

Now try same balloon with a thicker layer of tire rubber..
Add in extra cords and steel belts too.
Probably couldn't inflate...would take way more pressure than you can generate..
Simple as I can explain it.

 

best example i can come up with is a tractor trailer tire

we run pressure like 105 in the steers and 95 in the drives/ tandems

these sidewalls are thick and the tires hold a lot of air.. but if you ran 40 psi just on the fact of volume and side wall strength, the tire would never stay on the bead.. in fact usually anything below 75 psi, a lot of times the tire falls right off and doesnt even need weight on it..

unlike my 44s where the max pressure is 25 psi. (being bias ply) the sidewalls are where the strength is but flexibility is also required. the volume of air is enough to hold the tire on and the vehicle up no problem. this tire and the tractor tire is actually of the same size but different purpose. TT tires roughly measure 40"-44" tall.

different applications require different tires and different designs.

just like you wont be able to take your geo metro tire and use it to hold 80k lbs...no matter what psi you put in the tire.

 

More weight that you carry the more air pressure you need...E rated allows for that. It has nothing to do with the weight of the truck but more to do with the weight your truck carries. Tire manufactures suggest a max pressure based on what there tires can handle.
I ran 35 in my K02's but when I towed I pumped the rears up to 44 to handle the added weight of the trailer ect.
Someone mentioned side wall flex...that can be scarry so make sure you adjust your pressure accordingly.

 

im just looking for a good explanation as to why this is the case

Imagine blowing up your typical party balloon with air pressure you generate. Not too hard?

Now try same balloon with a thicker layer of tire rubber..
Add in extra cords and steel belts too.
Probably couldn't inflate...would take way more pressure than you can generate..
Simple as I can explain it.

Sometimes the simple explanations are the best!
Very good.

 

the only issue with the balloon analogy is that you are not really inflating a tire to increase size like you do with a balloon. comparatively, the balloon is elastic while the tire is not. for rough math, a tire can be considered a pressure vessel. if you double the quantity of air molecules in a tire, you will double the pressure. this is not the case with the balloon as the balloon will expand. with an expanding balloon, the air is not compressing as it would in a fixed volume vessel like a tire.

the heat related response seems like the best explanation as heat is an enemy of tires. additional material flexing will create additional heat and hold more of the heat it produces.