Need PSI help.....

[97f150]

I recently put a new set of Nitto Terra Grapplers on my 02 Ram 1500. The size is P285/70/17. I went with P rated tires because I dont haul anything to require a D or E range tire, and frankly thats what the truck came with.

My problem is that these tires have a max PSI of 35. The door placard suggests 35 PSI. My issue is running these tires at max psi, temp flucuations, and because the tires have a higher load capacity at 35 then the ones I took off had at 44 psi concerns me about wearing the centers out. The tires I took off were from Firestones bottom shelf (Peerless Widetrack Baja H/T) and had a max weight/ psi of 2535 at 44 psi, the Terra Grapplers are 2833 at 35 psi, thus my concern. I ran the Widetracks at 40 psi and they wore as even as I could ask and with 42,000 miles on them still had 6/32 (which is half) of tread left and i now in service on a co workers truck.

I called Nitto and they looked up what Dodge puts on the placard and told me to run it at that PSI totally ignoring the concerns I had about the load capacity and over pressurizing the tires once they heat up. Is there a chart, or any expert advice that I can use as a guide, maybe a weight to PSI conversion or something. Thanks in advance.

 

[shanneba]

Maybe this will help:

Firestone Tire Guide

 

[simple_gifts]

OT: I have to remember the difference between 97f150 and 97tbird; I'm sitting here perplexed by this post wondering what happened to the BMW..... Time for bed I guess.

 

[kr_bitog]

Originally Posted By: 97f150
I recently put a new set of Nitto Terra Grapplers on my 02 Ram 1500. The size is P285/70/17. I went with P rated tires because I dont haul anything to require a D or E range tire, and frankly thats what the truck came with.

My problem is that these tires have a max PSI of 35. The door placard suggests 35 PSI. My issue is running these tires at max psi, temp flucuations, and because the tires have a higher load capacity at 35 then the ones I took off had at 44 psi concerns me about wearing the centers out. The tires I took off were from Firestones bottom shelf (Peerless Widetrack Baja H/T) and had a max weight/ psi of 2535 at 44 psi, the Terra Grapplers are 2833 at 35 psi, thus my concern. I ran the Widetracks at 40 psi and they wore as even as I could ask and with 42,000 miles on them still had 6/32 (which is half) of tread left and i now in service on a co workers truck.

I called Nitto and they looked up what Dodge puts on the placard and told me to run it at that PSI totally ignoring the concerns I had about the load capacity and over pressurizing the tires once they heat up. Is there a chart, or any expert advice that I can use as a guide, maybe a weight to PSI conversion or something. Thanks in advance.

If the door placard shows 35 psi for same P rates tyre size and load index, I would not worry to put 35 psi first. If you find wearing on the center, then you can slowly adjust lower a bit. What I find the tyre pressure and load is not really linear, so it is difficult to compare different brands even though the load capability should be similar based on the load index.

 

[97f150]

I put around 1500 or so miles on the tires since they were installed last week. I ran them at 35 psi since installation and just took a tread guage and checked the depth across the tread. I did this on all 4 tires and they all read the same, so no indication of them wearing one way or the other yet.

 

[Colt45ws]

Standard Load P tires hit maximum load at 35psi, even if the tire says 44psi, by the way. Xtra Load tires hit it at 41psi.
Also, the max is for cold. The extra pressure from heat is accounted for in the tire design.
P tires are de-rated in a light truck application by 10% as well.
So its calling for 2280lbs/tire.
So your old tires have a 113 load rating and the truck wanted all of it.
Your new tires have a 117 load rating. You can run them at 29psi for 2623lbs (2370lbs derated)...

 

[CapriRacer]

Amateurs! Sheesh!

First, a bit of theory. You want to match the load carrying capacity of the truck at the conditions specified by the vehicle manufactirer - so you start with the vehicle tire placard. You spent a lot of typing telling us about the pressure and what's written on the sidewall of the various tires involved - but you didn't tell us what the original tire size was.

If my book is correct, your vehicle tire placard will say the original tire size was P265/70R17 (mounted on 8" wide rims!). At 35 psi, a P265/70R17 has a load carrying capacity (on a truck) of 2305#. In order to carry the same load, a P285/70R17 needs to use 28 psi.

But you said you ran the P265/70R17's at 40 psi - and here's where it gets a bit dicy! In theory, the load carrying capacity of a Standard Load Passenger Car tire maxes out of 35 psi - but the size and shape of the footprint don't know this, so they will change with increasing inflation pressure.

If we assume that the load carrying capacity COULD increase (in theory), then at 40 psi the load carrying capacity would be 2470#. In order to carry the same load, a P285/70R17 needs to use 32 psi.

But there is a school of thought that says that once you have enough load carrying (by pressure) in your tires, the next thing to be concerned about is spring rate - which is also closely tied to inflation pressure. That is probably where the Nitto folks were coming from.

Oh, yeah! One more thing to check - rim width. A P285/70R17 has an allowable rim width range of 7½" to 9½", so that works as well.

That's great for theory, but how well does this work in practice - and what about your concern about wear.

Tire wear is very complex and trying to sort out all the variables is difficult. While inflation pressire plays a role in how even a tire wears, it can be completely overwhelmed by the affect alignment and cornering have. (Most tire wear occurs in the cornering mode!) This is where the "match pressure" school of thought comes in.

Matching the pressure matches the spring rate, and that makes the tire behave as closely as the original does when it comes to how the vehicle behaves relative to the road surface (bumps, etc.) - and from a safety point of view, this is more important than how the tire wears. Besides, it is quite likely that the tire wear will not be significantly affected by the pressure - and that makes "matching the pressure" a viable theory.

And to correct 2 misconceptions: What is written on the sidewall of the tire for pressure may or may not be where the load carrying capacity maxes out. If it says "Max load XXXX at YY psi", then that's a relationship, but most tires will say "Max Load XXXX, Max pressure YY psi." - which isn't stating a relationship at all!

Further, any concern about using an inflation pressure nearer or at the maximum is misplaced. It's really hard to say this in simple terms (because the issue is quite complex), but the short version is that there is enough over-design in tires that this should not be a concern.

 

[97f150]

Originally Posted By: CapriRacer
Amateurs! Sheesh!

First, a bit of theory. You want to match the load carrying capacity of the truck at the conditions specified by the vehicle manufactirer - so you start with the vehicle tire placard. You spent a lot of typing telling us about the pressure and what's written on the sidewall of the various tires involved - but you didn't tell us what the original tire size was.

If my book is correct, your vehicle tire placard will say the original tire size was P265/70R17 (mounted on 8" wide rims!). At 35 psi, a P265/70R17 has a load carrying capacity (on a truck) of 2305#. In order to carry the same load, a P285/70R17 needs to use 28 psi.


Your book is absolutly correct sir! The only thing that differs is that the rim is 7.5" wide. The door placard actually has the tire listed as a P245/70/17, but the tire size that the truck was delivered with (evidenced by the build sheet) was P265/70/17.

Originally Posted By: CapriRacer
But you said you ran the P265/70R17's at 40 psi - and here's where it gets a bit dicy! In theory, the load carrying capacity of a Standard Load Passenger Car tire maxes out of 35 psi - but the size and shape of the footprint don't know this, so they will change with increasing inflation pressure.


This just about clears up everything I need to know. Anything after 35 psi is strictly to control tire wear, what really matters is that the tires be inflated to at least 35 psi. Just to clarify one more issue I have. I adjusted the tire pressure night before last, the temp was 68 degrees. So the tires have 35 psi in them at 68 degrees. During yesterday the temp went up to almost 90 degrees. So, assuming (at least this is what I have always heard) that for every 10 degree increase/ decrease in temprauture, will result in a 1 psi increase/ decrease in pressure. So if my truck just sat still from the night before, my pressure could have risen up to 37 psi before even driving. I know that the tire is built to handle the added heat and all that, but assuming the situation I just presented, the tire would be over inflated before driving the first foot. Any ill effects to this?

Thanks for all the help guys. This is the 4th place I post this, and the only place I have received anything besides opinions.

 

[CapriRacer]

Originally Posted By: 97f150
.....Your book is absolutly correct sir! The only thing that differs is that the rim is 7.5" wide. The door placard actually has the tire listed as a P245/70/17, but the tire size that the truck was delivered with (evidenced by the build sheet) was P265/70/17.........


I know that Dodge does some peculiar things with their vehicle tire placard, but I was under the impression that they are required by law to list the original tire size - and the way I've seen Dodge do that was with an additional sticker nearby (and sometimes over) the other one.

Also, are you sure the rim width is 7½"? My book lists only 8" supplied for your model year, but does list 7" starting in 2005. No 7½" listed anywhere! Is it possible the dealer changed the tires? - and the rims? It's not like the book is infallible, but it's right a lot!!

The reason this is important is that IF your rims are too narrow, that will have affects on tire wear.

Originally Posted By: 97f150
.....This just about clears up everything I need to know. Anything after 35 psi is strictly to control tire wear, what really matters is that the tires be inflated to at least 35 psi. Just to clarify one more issue I have. I adjusted the tire pressure night before last, the temp was 68 degrees. So the tires have 35 psi in them at 68 degrees. During yesterday the temp went up to almost 90 degrees. So, assuming (at least this is what I have always heard) that for every 10 degree increase/ decrease in temprauture, will result in a 1 psi increase/ decrease in pressure. So if my truck just sat still from the night before, my pressure could have risen up to 37 psi before even driving. I know that the tire is built to handle the added heat and all that, but assuming the situation I just presented, the tire would be over inflated before driving the first foot. Any ill effects to this?

Thanks for all the help guys. This is the 4th place I post this, and the only place I have received anything besides opinions.


If you read my post again, you'll see that pressure doesn't do much with regrads to tire wear - at least in comparison to other things.

Also, the change of a couple of psi isn't significant for other properties as well. But large changes are!

And lastly, where else did you post this question? I am always looking for sites where there are tire discussions.

 

[97f150]

Your book is correct. I went back and checked and the rim size is 17x8, not sure where I got 7.5 from, I just knew the tires would fit lol. The door placard actually lists 2 tire sizes..............and they are the same, I will take a picture of it later if I have a chance. The build sheet says it left the factory with P265s though, and thats what was on the truck when I bought it.